Exactly. It's like a golden opportunity for me to demonstrate some engineering chops and instead the interviewer expects me to just whip out some caching implem

Exactly. It's like a golden opportunity for me to demonstrate some engineering chops and instead the interviewer expects me to just whip out some caching implementation for some nebulous reason because that's what they would have done. If there's a real problem with either of our naive implementations the proper thing to do is to tell us what the problem is and leave us free to solve it.