An interview process is (ideally) about team fit as much as technical prowess.

Technical ability is not the only requirement for most jobs.

Fair enough, but why the leetcode-style interview then?

Honestly I've never met anyone who really likes them, but people want to simulate a situation where you're working together to solve a technical challenge, and see how you work with others, how you clarify requirements, how you communicate progress and ideas, how you test your hypotheses.

Sometimes people really do just want to see if you've memorized Cracking the Coding Interview, and that sucks, but often it's more "can you work out a reasonable solution to this problem in a reasonable way, and tell me about it". We don't know the details here, but I'm guessing all this company said was "four interviews, and one of them will be a technical whiteboarding exercise". I believe you can still get a lot of insight out of a whiteboarding session even if you know for a fact the candidate can write good code.

There are other options, like "tell me about a challenge you've faced" or just talking through a particular project on the resume, but the interviewer might want to have actual code to talk about, because that's a particular set of problem solving and communication skills that another conversation might not get at. I think that's legitimate.