I learned about that proof from a YouTube math video, rather than my Computer Science degree or minor in Mathematics.

People expect genius in interviews all the time. They just don't realize that's what they're doing. They think what they're asking about is an obvious concept, forgetting that (insert renowned genius here) came up with the idea.

I'd have thought group theory would be a required math course. This would also come up in Comp.Sci. complexity contexts and even in Calculus or mathematical logic contexts (I'm sure in many others I'm missing). Way back when this was a required first year course in the Comp.Sci./Math program I took.

I suspect you are forgetting the possibility they just didn't cover that particular proof.

I also would have thought that it would be covered in a CS curriculum, as I said. it'snot just some random proof, but an important foundational one. (I don't think I "forgot a possibilty", I just thought it usually would).

You are saying you took classes that covered logic, number theory, group theory, algorithmic complexity, discrete math, and calculus, and you are certain none of them covered this? Too bad, that is unfortunate! I'm glad you found it on your own! it's really neat!

Yeah. It's pretty foundational. e.g. https://en.wikipedia.org/wiki/Aleph_number I guess it's possible to teach it without going through the proof.

How many times have you been asked to rediscover the Fibonacci sequence and then make it useful to some stupid business?

Yes. Cantors diagonalisation wasn't just a neat proof for an interesting theorem, but it made Cantor one of the few people to invent an entirely new class of proof technique. I'd give it a similar status to techniques like 'indirect proof' and 'induction'.