The seed of Buddhahood is Myoho renge kyo [Lotus Sutra].

“The meaning of this passage is that those who obtained benefit during the Former and Middle Days of the Law received “conspicuous” benefit, because the relationship they formed with the Lotus Sutra during the lifetime of the Buddha had finally matured. On the other hand, those born today in the Latter Day of the Law receive the seeds of Buddhahood for the first time, and their benefit is therefore “inconspicuous.” — Nichiren from Teaching, Practice, and Proof

SGI background states:

“..In this letter, Nichiren Daishonin states that Shakyamuni’s teachings no longer lead to enlightenment in the Latter Day of the Law, and that only Nam-myoho-renge-kyo combines the three elements of teaching, practice, and proof essential to making Buddhism viable.”

An SGI member regurgitates this misunderstanding:

“In this passage Nichiren states that those who lived in the Former and the Middle day of the Law were able to attain Buddhahood through their relationship with the Lotus Sutra of Shakyamuni and his disciples who have been born together, their cause have matured as the effect accumulated from past existences therefore their harvested benefit is conspicuous since they share the same heriage. However, those who are born in the Latter Day of the Law are no longer connected to Shakyamuni’s harvested enlightenment and are receiving the seed of Buddhahood hidden in the depth of Lotus Sutra for the first time through the very Law that all Buddhas have attained throughout time and space revealed by Nichiren Daishonin being Nam-myoho-renge-kyo.”

Nichiren, on the other hand, goes on to correct this non sequitur:

"Answer: The above-mentioned commentary states that such an interpretation of the last five-hundred-year period is “superficial.” From a more profound viewpoint, the Lotus Sutra will spread throughout the ten thousand years of the Latter Day. The Great Teacher T’ien-t’ai comments on the previously quoted sutra passage, stating: “It is not only the people who live during the Buddha’s lifetime who obtain great benefits. In the last five hundred year period, the mystic way will spread and benefit humankind far into the future.” Does this annotation suggest anything other than the ten thousand years of the Latter Day of the Law? The “Distinctions in Benefits” chapter in the sixth volume of the Lotus Sutra states, “In the evil age of the Latter Day of the Law if there is someone who can uphold this sutra . . . .” Also the “Peaceful Practices” chapter reads, “In the Latter Day of the Law, if one wishes to preach this sutra . . .” These quotations refer to [the propagation of the Lotus Sutra in] the ten thousand years of the Latter Day of the Law. All the Buddha’s teachings other than the Lotus Sutra are covered by his declaration: “In these more than forty years, I have not yet revealed the truth.” Moreover, there are some cases where the sutras have been revised according to the understanding of those who compiled them and therefore cannot be trusted.”

SGI and the above mentioned SGI member have revised the teachings and can not be trusted. Let us look at the most important Gosho, The True Object of Worship to see if what SGI teaches accords with the mind of Nichiren:

“People can attain enlightenment in two ways: by meeting the Buddha and hearing the Lotus Sutra, or by believing in the sutra even though they do not meet the Buddha…” [Writings of Nichiren Daishonin page 359]

Nichiren goes on to say:

“It is evident that the Revelation of Eternity was preached for the sake of those who live at the beginning of the period of Deprivation. In this respect, we are as fortunate as those who heard the Buddha in person, the difference is, they brought to fruition Buddhahood by hearing the ‘One Chapter and Two Halves’, while we are endowed with the seeds of Buddhahood by receiving the Five Words.” (Five Major Writings of Nichiren, Kanjin No Honzon Sho, NOPPA pg 370)

It is always the Lotus Sutra that leads to Enlightenment but the practice of the Lotus Sutra differs according to the age. The seed of Buddhahood is always Shakyamuni Buddha's Myoho renge kyo [Lotus Sutra].

The practice of the Lotus Sutra in the Latter day is Namu Myoho renge kyo, not singing silly songs to Sensei or deprecating the Lotus Sutra by teaching that it is no longer valid in Mappo.