The correct answer is “Postgres would handle it, but if it needed to scale even higher, I’d…”

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

And people weeded out by this kind of questions are probably rightfully so. I for one could not ever work with someone that says "my answer is correct, period.". Part of the answer and the discussions made by mature individuals must ask for feedback, incorporate it in your design, be open to compromises sometimes but also to die on a hill when it makes sense. And in an interview context, you ought to show the hiring manager all these faces.

Then, there are hiring managers that suck and you might be discarded because you didn't follow the script. Sure, but that's a bullet you dodged.