Not "this Myoho renge kyo is not Shakyamuni's Mystic Law" but rather this Ongi Kuden is not Nichiren's teachings

The Ongi Kuden states:

"This Myoho-renge-kyo (Lotus Sutra) is not Shakyamuni's Mystic Law, because at the time this chapter was preached, he had already entrusted it to Bodhisattva Jogyo. (Gosho, p. 1783)

But the Kanjin Honzon Sho (The True Object of Worship) states:

"Having thus manifested the ten divine powers, Sakyamuni Buddha transmitted the five characters of Myo, Ho, Ren, Ge, and Kyo to his original disciples since the eternal past, who had sprung up from under ground." Kanjin Honzon Sho, pp 122 to 140, NOPPA 1991.

and the Lotus Sutra:

"At that time the Buddha spoke to Superior Practices and the others in the great assembly of bodhisattvas, saying: "The supernatural powers of the Buddhas, as you have seen, are immeasurable, boundless, inconceivable. If in the process of entrusting this sutra (Myohorengekyo) to others I were to employ these supernatural powers for a measurable, boundless hundreds, thousands, ten thousands, millions of asamkhya kalpas to describe the benefits of the sutra, I could never finish doing so. To put it briefly, all the doctrines possessed by the Thus Come One, the storehouse of all the secret essentials of the Thus Come One - all these are proclaimed, revealed, and clearly expounded in this sutra (Myoho renge kyo). -- Lotus Sutra Chapter 21

Not only is the Ongi Kuden wrong in light of the Lotus Sutra and the the most important authentic Gosho, it makes no sense.