Yes, and I would because if I don't know then that's my only option (short of sitting there and going "no can do"). However, given their phrasing "basic shit like factorial or fibonacci" I got the vibe this is supposed to be known by the candidate and they'd judge the candidate negatively (before having written a single line) for needing to ask

I'd assume that the purpose of a simple question like that is to test basic programming skills (can you write a loop?). Testing knowledge of a specific math concept is doesn't really fit into this goal, so you're unlikely to be penalized for it.

You are correct.

We do give the definition of Fibonacci, and we steer the candidate in the right direction if they make math errors.