The point is the interviewers are sometimes obtuse.

Sometimes the point of the interview is to see if the candidate knows an existing solution and "just use postgres" is the good answer. Sometimes it's to test the technical chops and pretending postgres doesn't exist is the point.

The candidate shouldn't be expected to always guess right, unless the position says "a psychic". The interviewer should notice if the candidate is solving the wrong kind of problem and nudge them in the right direction instead of punishing the candidate for the wrong guess.