I wanted to call out a couple of points at first.

1. It is still a startup and doesn't know how to interview people.
2. The interviewer simply throws a problem and watches the fun rather than understanding the code or providing hints to assess the candidate.
3. The interviewer expects you to run your code on HackerRank and also expects you to write unit test cases.
4. Given the one-hour time, it's not possible for someone to write working code along with test cases.

Stay away from this company until they are mature enough and understand how to conduct interviews.

I work at FAANG, have interviewed hundreds of candidates, coached many engineers on how to interview, and also attended 50+ interviews in my career. This is the worst experience I've had.

Implement an API rate limiting program.