My understanding is that they are actually talking about both. This is also a stance that could be supported by the “closely linked” quote I’m sure you’re aware of.
This is another quote from the same thread just to illustrate MMR is undeniably being discussed:
So then why do points for losses and wins seem so random? Well, the amount of MMR (and SR) you go up or down isn’t simply a matter of whether you won or lost, and what was your predicted chance of winning. There’s a couple of other things at work. One is the matchmaker’s confidence in what your MMR should be. Play a lot of games, it gets more certain. Don’t play Overwatch for a while, it gets less certain. You go on a large win or loss streak, it gets less certain. The more certain the matchmaker is about your MMR, the less your MMR will change in either direction based on a win or loss.
If you put all of their sources together from the past, we would largely be led to believe that for most of the population that are operating under normal conditions (active accounts, no large streaks in recent history, etc.), their MMR and SR would be close enough to one another that they would essentially exist as being the same value.
As for how that is in the current game with everything that’s changed or how the latest post on matchmaking reads… I honestly don’t know what they’ve changed or have any worthwhile amount of confidence to argue too much on matchmaking topics.