MHz’s theories are even worse than Cuthbert’s. At least Cuthbert took a Blizzard statement, but twisted the hell out of it to fit his narrative. MHz just completely made BS up and claimed he “tested” them.
Everything I know about Overwatch’s matchmaker in particular comes from Kaawumba’s topics, and he’s compiled every single Blizzard statement on matchmaking into his references section:
The rating and matchmaking system is confusing, and a good overview does not appear to be available. The official overview (1, 48) is incomplete and does not answer a number of common player questions and concerns. This information below is gathered from sporadic blue posts and developer update videos, and salted with my own experience and experiments, various forum threads, and watching streams. Note that since Blizzard does not give exact algorithms, I do have to fill in some gaps, or leave so…
I decided to use the line that “MMR basically translates to your SR” because of these two Jeff Kaplan posts
Overwatch Forums
Overwatch Forums
In particular, Jeff states that unless your SR decayed due to inactivity (above diamond), your MMR and SR are “closely linked”.
Also stated is that MMR works similarly to SR, and that it going up or down is contingent on winning and losing: