Not exactly the same situation, but from a snarky interviewer I've gotten the answer equivalent to
"lol I was told you were the Django expert? According to your resume? Are you not supposed to know that?"
Still it does tell you that you have to work with this massive asshole, so it's a worthwhile approach to get more info about the workplace you'll be in.
> get more info about the workplace you'll be in.
or about the bullet you just dodged...
if someone connected django with number theory in their head and assumed everyone else can see the connection just as spontaneously, i'd much rather leave than try to handle that arse.
I would add here, that you may ask in a polite way, how number theory relates to the position at hand, here Django. This way, the interviewer has to open up specifically. There might be something, that could be important and interesting or simply a bluff.