@Sand_spitter as I see it SBMM works something like this:
Say we have 2 servers and 150 players online at a time.
SBMM takes all 150 players and rank them from 1 to 150 "place/rank", and then just puts first more similarly skilled 60 into server 1 and next 60 into server 2 and then leaves 30 people waiting, can be either case the most skilled or least skilled ones.
Potentially there are some rules that if your skill gap is much higher than currently available players, maybe a waiting time of X minutes is applied, hoping that more skilled players will come online - explains why Diamond+ players usually wait quite some more to get into the game.
I guess we assume that "skill-based" equals "equally skill-based" which is not quite true, the same.
E.g. not skill based or differently named matchmaking would be ELO based or just first come first served based.
Honestly, as a new player, I feel a bigger gap in higher lobbies vs lower - I did not mind losing a lot while being <Gold, yet having no chance in Platinum (currently full of ex diamond+) is a bit frustrating.