So, it sounds like the candidate was interviewing for a senior engineering position and was given a problem to reverse a linked list in place. This is an entirely reasonable constraint and something that you would expect someone in a senior position to be able to do.

Space constraints do come up frequently and being able to reason about them is important.

I still maintain that asking for specific answers is a bad interview practice. It's a sign of a poor interviewer, not necessarily a bad candidate. It could be both in this case, but I'm talking about it at a more meta/higher level. IMO, it is objectively BAD interview practice being described here.