The difference is on the margin. If typical candidates who reach the optimal solution reach it in the last 5 minutes, and if it takes 5 minutes to discuss with a non-chess-playing candidate how a knight moves, then you have severely disadvantaged that candidate.
Isn't this somewhat dependent on the outlook of the interviewer? I know interviewers will greatly vary, but is it not more important to consider what my path to the solution was, rather than what the actual answer is?
I would be more inclined towards a candidate that knew nothing of a problem but was able to explore a way to the answer vs. a candidate that knew the answer simply because of hours of rehearsal
