However, it is a failure mode that people who are really smart and qualified about one thing can assume they are equally smart and qualified about a bunch of other fields that require their own specific expertise. Alas, it doesn't work that way.

There's nothing wrong with not knowing about some specific technical subject. It is a red flag though when someone takes it as a threat to their identity and self-worth to acknowledge that they don't know much about some particular subject, even if they do know a lot about a different subject.