Obviously. But they don't care about that answer, they care about the design of a new system so I'm saying the interviewer is dumb for not just asking directly for the solution he wants to see (while also giving credit for the original right answer).

The interviewee is dumb (if he wants the job) to not know this is what they want to hear, so he can just say "I really do think Postgres would satisfy the requirements here, but as an alternative a custom solution would look like.."

What is the point of not demonstrating your own well rounded knowledge when given the opportunity? Is it not obvious what they want to hear? Then why not tell them, again given that he did want the job

The point of a system design interview is to have a discussion that examines possibilities and tradeoffs.