Fair enough, I did skip over that last line you typed the first time I read through it before replying.
All Blizzard statements throughout this forum, Twitter, etc. has been compiled in Kaawumba’s topic as I linked above.
In particular, to note, MMR is the sole criteria of how people are matched (reference 25 in Kaawumba’s references list towards the very bottom of his opening post), and SR being a more “digestible” form of MMR (reference 22).