At my last employer there was a similar story. The candidate mentioned he had written a book on Flask. So the dev in the interview took that as an invitation to skip the warm ups and go on a deep dive into Flask internals. The author got extremely flustered and couldn’t answer.
I don't think this matters. If he strongly claims he is an expert on Flask, and I knew Flask, then I would test the depth of that knowledge. You can't validate every detail on their resume, but if you find they misled about this one fact, odds are they mislead on other facts you are not capable of validating.
