Okay, I guess I have been in a position to interview senior software engineers who can "design new systems" all day on a whiteboard with artificial "interview"

Okay, I guess I have been in a position to interview senior software engineers who can "design new systems" all day on a whiteboard with artificial "interview" constraints but who could never actually solve this problem. I think this is a better question for senior engineers than junior ones. You seem to be assuming that a senior candidate who could blag their way through a "hard" design question in an hour could ALSO solve this problem and I don't know where you've worked but in my experience that is not at all true. In my experience, senior engineers often can't code much at all because they have worked at places with a ton of people writing code where code just "happens" and they are good at all the other processes that are code adjacent but "I haven't written much code recently". You better test that they can write real code and not leetcode. I'm a senior engineer and I do a lot of design and architecture and code review every day but I would probably fail most generic leetcode tests and I think I would pass this one, so I feel kind of personally validated by that, so excuse the passionate response. :)