"The problems were so difficult that he wanted to sit with me and read them together to make sure that I understood them"

That sounds slightly condescending to me, especially as the problems don't sound that complex to describe and he is a mathematician! It doesn't surprise me that someone who takes that attitude reacts badly to someone easily solving a problem that they couldn't.

I'd suggest more that the interviewer felt threatened rather than envy. From their point of view, they would have someone smarter than them working under them, who might show them up to senior management or displace their current position in the company.

Because being able to solve puzzle problems isn't actually a good predictor of employee quality? (It remains an open question why interviewers use them at all in that case.)

Many interviewers do not (in reality) want to employ people who are obviously better than them, whatever they may say in public?