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Have you read 'Surely You're Joking Mr Feynman'? Dude was a youtuber physicist in a world where youtube didn't exist.

Somebody put his lectures up on YouTube a long time ago.

If "person who's content is available on YouTube" qualifies one as a YouTuber, then yes

I think generally "YouTuber" is taken to mean somebody who is building particular YouTube personality/brand thing.

But even if we want to be very general and just parse it using default English, the -er suffix would mean someone who is doing something. I believe it would be more accurate to say Feynman was YouTubed.

Feynman most probably would be a youtuber if he were alive.

Even posthumously he’s been one of the most popular educators on youtube.

How did you go from "Feynman Lectures Book III" to "Youtube"? billfruit was asking about a book.

Let me answer for OP. Because all these lectures are actual recorded lectures that have been uploaded to YouTube.