I've been coding for over a decade including things 1000x as complicated as that (think embedded systems from the ground up). I could easily take 20 minutes or even fail doing such a task because I'm literally never asked to code things in a professional environment with 3+ strangers breathing down my neck and judging me based on some simplified fizzbuzz.

I'm convinced white board / live coding interviews don't do much other than test for how you perform with a group of strangers pressuring you to jump through hoops publicly on which in a short time your entire value is judged (including determining say if you'll have money for daycare next week), which for most software professionals basically happens never except during an interview.

I could bikeshed on this, but I think this is missing the overall point. It's not that nothing is measurable, obviously some tech skills are measurable and obviously some people are going to do better on those things, but those things are less measurable as you move up the ladder, and less measurable for leadership skills over technical skills, and less measurable among groups of people with relatively similar levels of experience. I'm not saying nothing can be measured so you might as well hire the mailperson to the principal engineers job. The point is that you can have a fairly large group of equally qualified people for a job.

20 minutes might just be interview anxiety, especially if it’s a first question in the interview.