I pretty much agree, especially about the self-selecting part. Girls at a young age tend to be more self-aware. Boys, not quite as much. (Not scientific either, just based on observation.) My concern would be that you get two or three high energy boys together at a Brownie troop because their parents can’t find any Scouting troop nearby and then the girls start dropping out because they no longer feel comfortable.
Sure, maybe the better distinction would be high-energy vs low energy. There are plenty of high energy girls that don’t do well in Guides. But, there’s no easy way to make this division. And gender is at least fair 95% of the time.