Myth.
MMR is determined by statistical performance
The idea here is that a player’s MMR is really a summary of their statistical performance, and that because SR chases MMR, a player’s SR is basically determined by their statistical performance. If this were true, statistical performance would be, by far, the most important contribution to SR. This is contradicted by Blizzard and by most players having a win percentage of about 50% (if statistical performance was weighted very heavily, stat-chasers would have much lower win percentage at a given SR than team-players, for example).