I feel like the point of the comment you're responding to (which I mostly agree with) is not so much that all the coaches select based on the letter leagues, but rather that you need to be a high-level club player (or equivalent) to make the team. I mentioned before that some of the kids at my son's school, who were playing F2 club soccer, did not make any team (not even the Frosh team), because they were not good enough. I think it's certainly possible to make a HS team without being in a prestigious league, but the chances of a kid being a top-tier player and not playing on a high-end club team already is probably pretty small.
I think it's dependent on the sport and the school, but it certainly feels like SoCal schools have become a pressure cooker for popular sports, where kids need to be high-end skilled already to make any popular sport teams at the school. This is certainly not exclusive to soccer by any stretch, though (my son's middle school had eight boys basketball teams one year, for example, due to their policy of not excluding any interested kids from sports where possible, and interest levels; only two competed, but they had that many extra kids trying to play). It's not really possible to be a recreational player and make a HS team any more, for those sports where the school is competitive.