This absolutely wrong. Unfortunately some people spread this misinformation. As a sad attempt to justify their rank to themselves as being not their fault. But let’s not dwell on that.
The matchmaker exclusively matches based on MMR. This has been verified by official Blizzard employees multiple times. Here is just one example: (note it is from a while ago before you could drop tier icons).
https://twitter.com/playoverwatch/status/850435344457543680?lang=en
It’s also easily demonstrated in game. If a GM player decays to 3000 SR, they will be put right back into GM games even as “a diamond” because their MMR is unchanged. Likewise, in the past placements used to lower your SR, but not your MMR. Thus you could see the exact same effect at all ranks, not just above 3000SR.
The “mystical matchmaker” people talk so much about is just one number for each player: your MMR. It goes up when you win, down when you lose, with the amount changed based on a few factors, but those are just some details. All the match maker does it try to get as close of MMR between people in a game as possible, because that is a balanced, fair match.
Once you understand how the system works, more or less the same as classic Elo, you realize why all these crazy conspiracies about things like “forced losses” are wrong.
For more details and citations about the match maker, see Kawumba’s excellent thread: