"Nichren Daishonin wrote more than a thousand letters and treatises but only 600 remain", ... Nagashima implying that we don't know everything that the Daishonin taught, therefore, the way SGI teaches is the way Nichiren taught. First, this is the logical fallacy Ad ignorantiam:
"The argument from ignorance basically states that a specific belief is true because we don’t know that it isn’t true. Defenders of extrasensory perception, for example, will often overemphasize how much we do not know about the human brain. It is therefore possible, they argue, that the brain may be capable of transmitting signals at a distance." -- Top 20 Logical Fallacies and this just happens to be number 1. [from The Skeptics Guide to the Universe].
More importantly, Nichiren Daishonin teaches,
"I would reply that, by looking at one thing, you can surmise ten thousand. This is what is meant by the statement that you can come to know all under heaven without ever going out of your garden gate. But a fool will have doubts, saying, “I have seen the sky in the south, but I have not seen the sky in the east or west or north. Perhaps the sky in those other three directions has a different sun in it from the one I know.” Or he will see a column of smoke rising up beyond the hills, and although the smoke is in plain sight, because he cannot see the fire itself, he will conclude that the fire may not really exist. Such a person is my questioner, an icchantika, or person of incorrigible disbelief, no different from a man with sightless eyes!"
Therefore, when we see one of Nichiren' Daishonin's clear statements about the identity of the original Eternal Buddha [Shakyamuni Buddha] from one of his most important writings, we know that he taught the same thing everywhere or we can see one hundred and fifty clear statements from the remaining Goshos and know what he taught in the missing Goshos.
If you should happen upon this lightweight's lecture, please also notice that he says nothing about Goshos that have proven to be forgeries, many of which are the very foundation of the Gakkai's phony Buddhism.