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.