Or just add a couple zeros to all the requirements until postgres is a worse solution than whatever the interviewer envisions. Isn't that the point of stating throughput requirements?

Right? I'll often structure interview questions like this. I give a basic problem, hoping for a basic answer. Then I add complexity, seeing how they respond.

In my experience, it's much easier to train somebody on how to scale a basic system up in response to need than it is to get somebody who favors complexity to cut it back.