That must be downright hilarious, depending on how the question is asked. Promises are an absurdly complicated construct considering how little they actually do. If you don't give the candidate the expected spec, 95% of candidates won't get even close. If you do, working through the answer will take long enough that the candidate (and honestly, the interviewer) might get bored.
If you simplify the problem space a lot and ask for a minimal "promise-like"-ish construct, then It can be an interesting coding exercise I guess.
Of course an hour long interview is not enough time to implement the full Promise spec but it should be enough time to gauge the level of a candidate.