If he were just stupid, I'd have no right to be indignant at his basic mistake.

As a general heuristic, if you agree that someone is both intelligent and highly educated, and has made a conclusion that you consider to be a basic mistake, there are a variety of reasonable responses, one of the most obvious of which is to question if the issue in question falls under the category of a basic mistake or even as a mistake at all. Maybe you should update your models?