This is absolutely, objectively false.
Spoken Q&A sessions are not neutral.
The origin of rhetoric is oral persuasion, not the transfer of objective information. Oral conversations are noisy AF, easy to dominate, allow speakers to lie with "I didn't say that", and to use persuasive rhetorical tricks to confuse and persuade instead of enlightening.
We've all been in meetings where people talk at each other, or everyone says their part with no connection to the rest, and previous points are forgotten almost immediately even if they're valid, true, relevant, and important.
Likewise lectures - good luck teaching anything scientific without allowing notes.
Writing gives the recipient time to stop, ponder, understand, and - on a good day - disagree and criticise.
It doesn't prevent questions at all. It encourages them, and gives them time to mature and deepen. You may not be able to ask the original writer - especially if they're dead - but you will always be able to find comments and opinions from others.
If you put all of these together you get an argument from status, not from effectiveness.
Being a teacher/philosopher was a career, and writing endangered that.
Superficial in-the-moment wit and memory were always better for entertainment than expanding human knowledge.