I understand that whiteboard can be frustrating and hard, but it sounds like your interview was the exact kind of interview people try to make
In the past when I've been the hiring manager, I've had inexperienced interviewers ask my candidates questions like this for all kinds of positions - when you look at who does well in other interviews and gets an offer (and performs well), it basically never correlated at all to performance in whiteboard-only CS-focused interviews.
In this case I was interviewing for a full-stack senior position where there would almost certainly be zero systems-level coding, so the interview should have been a throwaway at best.
Space constraints do come up frequently and being able to reason about them is important.