Except they’ve said the opposite. The reason they use MMR is to move it FASTER than they can move SR, especially on new accounts where there is high uncertainty.
It’s all in the guide. How Competitive Matchmaking and Ranking Works (Season 28+)
Give it a read.
Yes, new accounts with no history start in the middle, around 2350 SR, and SR is not used for matchmaking. They start at 0 MMR, with a high uncertainty, because there is no data for their skill. Please, just read the guide.