I think they went for the simplest to programme, ensuring they would never run out of Leaders for slots.
Maybe, without looking at the code (and having a lot more experience than I do), it would be hard to know for sure, but intuitively it doesn't seem much either way. One checks for the number of players that are going to be on the map and feeds that back to the leader pool selector to insist that you have at least that many, the other checks how many leaders are in the pool and then feeds that back to the player selection screen to insist that you don't have more than that. Plus, on the Switch (your video on the PC shows a much better version) they've made it counterintuitive in the UI - it's on the map selection screen, not the player screen where you select the various civs and leaders.
Without seeing the code, it won't be possible to know if the way they did it was mandated by existing code. Ignore the frustration, though - this is a feature that I've wanted since buying the game and I got very excited when I saw it, so to see it come but in this unwieldy and counterintuitive way is a bit frustrating. Still, I'm happy that we have it.
You don't need to select the pool for every included civ, so there should be no number requirement for the pool itself.
If the map size needs 8 and 5 are selected in the pool, then it should just become unavailable to select for the sixth spot.
That said, I'll probably only ever use it as Pool 1 for me and Pool 2 for all the AI.
That was my thought as well. It's not a case of disabling certain civs, so I'm not sure why it would insist that we do it that way. Either the code is really messy behind the scenes and forcing them to do it this way, or it was rushed and no one had time to really think about what was the best way and they just ran with the first thought that came to mind.
As you said, it's not like I'd want to use it in the way that it precludes very often so I'll live, it's just a bit counterintuitive and forces you to think more about it.