"The more certain the matchmaker is about your MMR, the less your MMR will change in either direction based on a win or loss.”
“Less” does not mean “doesn’t move much”. A million is less than two million, but a million is not a small number.
Comparing the movement of MMR in a single win or loss to movement of SR over many games is comparing apples to oranges. MMR is also expected to fluctuate widely over the course of many games.
Scott has said basically the same thing about SR, so your quote does not show any divergence between MMR and SR.