No exactly, i made a theory like you.
Though you’re saying i’m claiming things i don’t know whilst all you’re doing is theorizing too. So practically that goes against you.
The matchmaker was designed to try and create games with equivalent-sized groups, especially for solo players, and these statistics show that it’s doing a pretty decent job. We believe that games with equivalent groups create the fairest possible experience, and fair games create the best chance of players having a fun experience. When the matchmaker does compromise, it’s usually during off-hours when there aren’t as many available players, as well as at the lowest and highest extremes of SR.
Refer to: GROUPS AND MATCHMAKING IN OVERWATCH
However, when you decide to queue for a competitive game as a solo player, we don’t only place you in all-solo compositions. Sometimes, you’re in a 2,1,1,1,1 composition which has an overall observed win rate of 50.03%. Other times, you’re in a 2,2,1,1 composition with a 49.46% win rate, or a 3,2,1 comp with a 49.93% win rate. All of the possible combinations considered, the combined win rate for solo players end ups being 49.94%. That’s very close to the perfectly fair value of 50%, which means that having groups and solo players queuing together has nearly no effect on a solo player’s win rate.