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To me the interviewees didn't come off as you describe at all. They are discussing the subject matter, which is the focus of the book "craft of programming" (see: subtitle).

Also most of them (at least) are known to have a focus on the practice of programming itself. They are literally authors/stewards of established algorithms, programming languages, operating systems and browser engines. What else would would you expect?

I found them all to be really down to Earth for such accomplished people.

It's entirely a matter or perception (and prejudice, most likely).

For example, if you watch any talks by Joe Armstrong you'll immediately perceive it as a kind, funny and down-to-earth guy.

I listened to the audiobook and didn't get the same impression. I wonder if it's audio related.