David clarified for me by email:
In the end, the witness argument is a very weak argument, because who says that the Jews in the First Temple period thought of the revelation at Mount Sinai as a real event in the way it is described in the Torah? Maybe that was simply the language back then; that’s how they spoke, weaving God into every event.
Or perhaps through a long evolutionary process from myth to history, turning customs into historical stories—etiology: the plague of the firstborn explains why the firstborn are redeemed; the Exodus from Egypt on the fourteenth of Nisan (a random coincidence—specifically at the full moon and specifically in the spring month) explains why the holiday is celebrated in the spring with a historical coloring; and so on. Or the name Levi because “now my husband will become attached to me”—or better to say because the tribe members were Levites!! And an answer to the second part of the question is in the responsa.

I discussed the witness argument in The First Foundational Principle, in Truly, and in Unstable. In my view it is really not weak. I’m not fond of conspiracy theories in any field, and in any case, whoever raises them bears the burden of proof. After I reached the philosophical conclusion that God exists, the report that He revealed Himself sounds entirely reasonable to me, and therefore I have no reason to formulate conspiracies and “maybes” like these.