For Nichiren, the revelation of the original Buddha which takes place in the sixteenth chapter has two meanings: that of a theory of the nature of the Buddha, and that of a speculation on the situation of humankind. Śākyamuni’s “original causes and original results” become the turning point which allows postulating the contemporaneity of human beings to the Buddha. Human beings, in their spatial and temporal limitations, share the temporality of Śākyamuni in this world. Nichiren uses a theory elaborated by Chih-i according to which the past of Śākyamuni is reflected in the past of his disciples. This is the tie established, in the original time, between Śākyamuni and those who listen to the sutra or are willing to accept it. From here Nichiren draws the certainty of buddhahood for human beings:

We living beings of this land are since as many kalpas ago as five hundred particles of dust Śākyamuni’s beloved children. [The relation] between a Buddha with ties and the living beings [bound] by karmic ties can be compared to [the reflection of] the moon in the sky floating on clear water. A Buddha without ties in relation to sentient beings is like a deaf man listening for the sound of thunder or a blind man turning to sun and moon.

According to Nichiren, in the second section of the Lotus Sutra Śākyamuni speaks of this Sahā world as the original land, a pure Buddha realm compared to which the other lands of the ten directions are mere conventional worlds. In Chih-i’s exegesis, the “original land” is the land in which the original Buddha attained enlightenment, therefore the realm of only one type of Buddha. This “Sahā world of the original time” contrasts with the Sahā world where human beings live, which retains the characteristics of a “trace-land.” For Nichiren, on the contrary, there is only one Sahā world. Vulture Peak, the place where the Lotus Sutra is taught, represents both this world of ours and the most perfect world, the only possible “paradise.” There is no other reality, neither for humanity, nor for the Buddha. Whereas Chih-i apparently believed in the Western paradise of Amitābha and hoped to reach it after his death, Nichiren considered the assembly on Vulture Peak a symbol of those who, having received the teachings of the Lotus Sutra, are able to transform our Sahā world into a “resplendent land.” (See this blog post.)

In Nichiren’s hermeneutics the original land thus equals the human world. Since the world where humans live is also the original world in which the Buddha attained buddhahood, phenomenal reality becomes the ground of the most complete enlightenment, which opens to ultimate reality. This enlightenment of the Buddha in the remote past justifies the buddhahood of all beings of this world: Nichiren insists that the śrāvakas and pratyekabuddhas who are promised enlightenment in the first section of the Lotus Sutra could never in fact attain it if the original enlightenment of the Buddha described in chapter 16 had not occurred.
A Buddhist Kaleidoscope; Lucia Dolce, Between Duration and Eternity: Hermeneutics of the ‘Ancient Buddha’ of the Lotus Sutra in Chih-i and Nichiren, Page 232-233

Nichiren classifies all the Buddhas of sutras other than the Lotus and the Śākyamuni described in the first section and in the last six chapters of the second section of the Lotus scripture as temporal bodies, or “Buddhas of the Hinayāna.” Only the Śākyamuni who reveals his enlightenment in the past embodies the true Mahāyāna Buddha. To indicate the infiniteness of this Buddha, Nichiren uses the expression “without beginning and without end,” which properly belonged to a context related to Mahāvairocana Buddha and signified an existence not subject to temporal limitations. This expression suggests that Nichiren attributes an eternal nature to Śākyamuni, and at first seems to imply that he envisages a dharmakāya as the only ground of any reality. But Nichiren develops this infiniteness in a different direction.

Nichiren emphasizes that the Lotus Sutra is the only scripture where not only the dharma body, but also the recompense body, and the transformation body are presented as “infinite”: “When other Mahayana sutras speak of ‘without beginning and without end,’ they refer to the dharmakāya only, not to the three bodies.” Nichiren does not regard the distant past represented by the five hundred kalpas as a metaphorical image, but as a concrete reality identifying an active original body, a “Buddha who in the far distant past has truly manifested himself, has truly practiced, and has truly actualized his enlightenment.” Consequently, the meaning that Nichiren attributes to Śākyamuni is not symbolized either by a transcendental body whose existence is set in a world other than ours or by the recompense body of which Chih-i spoke. This “without beginning without end” of the temporal body is most difficult to believe, Nichiren repeatedly suggests, but the infiniteness of the nirmāṇakāya is the crucial evidence that the Buddha has always abided in this world and that his soteriological activity has been constant since the original time.

Thus Nichiren resolves the conflict between the mundane and the ultimate by creating an all-encompassing Śākyamuni Buddha, who maintains characteristics of the historical Śākyamuni (the activity of preaching) and at the same time is endowed with attributes of the dharmakāya (infinite existence). In this way, the dharma world itself comes to be conceived as the phenomenal reality which actualizes the ultimate truth. Borrowing from Tendai terminology, Nichiren calls this reality “a concretely accomplished ‘three thousand worlds in one single thought.’ ”
A Buddhist Kaleidoscope; Lucia Dolce, Between Duration and Eternity: Hermeneutics of the ‘Ancient Buddha’ of the Lotus Sutra in Chih-i and Nichiren, Page 231-232

In chapter [11] of the sutra, Śākyamuni’s emanations materialize, having been asked to gather from the ten directions. Chih-i had already suggested that the emanation-bodies of Śākyamuni prove that he had not attained enlightenment only forty years before preaching the Lotus Sutra, otherwise there could not have been so many kalpa-old beings who had received instruction from him. However, Chih-i did not invest Śākyamuni’s emanations with a universal significance, probably because he did not regard Śākyamuni as the only true Buddha of the universe. Nichiren’s declaration that all Buddhas enlightened in the past are emanations of Śākyamuni is of a different nature: it challenges the equality of all Buddhas and, furthermore, operates as a reduction which unifies all Buddhas, not only those appearing in the Lotus Sutra, but also those appearing in other scriptures of the Buddhist canon. It should be noted that this “absolutization” of Śākyamuni, although reminiscent of the idea that “all Buddhas are just one single Buddha” developed by esoteric Tendai in Japan, does not proceed by equating Śākyamuni with another Buddha already defined as universal, like Vairocana, but rather by including all Buddhas (Vairocana, too) in the person of Śākyamuni.

Nichiren discusses at length how all Buddhas are enlightened because of their relation to Śākyamuni Buddha.

If we consider the stage of results, the many Tathāgatas are Buddhas of a past ten kalpas, one hundred kalpas or a thousand kalpas long. Lord Śākyamuni is a Buddha who has [attained] the complete result of subtle awakening as many kalpas ago as five hundred particles of dust. The various Buddhas of the ten directions such as the Tathāgata Vairocana, the Tathāgata Amitābhā and the Tathāgata Bhaisajyaguru are followers of our original teacher, the Lord Śākyamuni. One moon in the sky floats in the water as ten-thousand [moons]. … This Buddha Abundant Treasures, too, is a follower of the Lord Śākyamuni of the chapter “The Long Life of the Tathāgata.

Nichiren’s interpretation of the Śākyamuni of the Lotus Sutra, although it took as its point of departure Chih-i’s theories, was definitively influenced by various hermeneutical patterns that developed in the Japanese exegetical tradition of the Lotus Sutra, and by Nichiren’s personal experience of the reality disclosed in the scripture.

Nichiren reread the entire sutra focusing on the “section of the origin.” From this perspective, he constructed an image of Śākyamuni Buddha as the only true Buddha of all Buddhist systems, and eventually produced an interpretation of the Lotus Sutra very different from that of Chih-i. In Nichiren’s writings we find a sort of dilation of the chapters constituting the second half of the Lotus Sutra, especially the end of chapter 15 and chapter 16, which Nichiren judges to be almost exclusively representative of the meaning of the entire scripture. This corresponds to the dilation of the temporal dimension expressed in those chapters, that is, the distant past in which Śākyamuni obtained his original enlightenment. Nichiren absolutizes this original moment and makes it the only significant time and relates it to the existence of humanity in a certain time and place.

He writes:

The true attainment of buddhahood in the far distant past is the original ground of all the Buddhas. To use a metaphor, if the vast sea is the true enlightenment in the past, the fishes and birds are the thousand two hundred and more Venerables. Had the enlightenment in the past not occurred, the thousand two hundred and more Venerables would be without roots like duckweed. …

When the past [of Śākyamuni] and [his] eternal abiding are disclosed, all Buddhas become Śākyamuni’s emanations. At the time of the earlier sutras and of the first part of the Lotus Sutra, the various Buddhas performed each practice and each discipline side by side with Śākyamuni. … Now it is manifest that the various Buddhas [of other sutras] all are followers of Śākyamuni. … When the Buddha is the Buddha of the far distant past, even the great bodhisattvas of the “trace section” and the great bodhisattvas of other realms are disciples of the Lord of the Doctrine Śākyamuni.