Whether this was even true in Feynman's time, it's much less true now.
With the meaning that Feynman meant it was true then and now. From QED:
>The next reason that you might think you do not understand what I am telling you is, while I am describing to you how Nature works, you won't understand why Nature works that way. But you see, nobody understands that. I can't explain why Nature behaves in this peculiar way.
Heresy! Feynman was the last Prophet of Physics, no one can ever figure out the things he didn't know. If people could discover things after Feynman, those things could get into textbooks, and then mere students could know things that Feynman did not -- which is clearly absurd.