My guess is that by the time the self-selection of people who think they're ready to apply and have a good enough shot to make it worth all the time the interviews take, happens, and they get to an in-person (having perhaps passed a less-harsh phone-screen leetcode question), most further leetcoding isn't doing much aside from keeping the self-selection effect strong.
The other parts of the interview might be doing something per se useful, I suppose.
But I think like you say, what they did do was create a filter that only very patient and technically qualified candidates could get through. Beyond passing the filter, their very standardized process did not derive any useful information about candidates. But I feel pretty certain if they made an offer to every candidate and not just those on the pass side of the filter line, they would have seen some differences.