The non TGM golfers set up a little different to the ball, not having the right forearm on plane. That means that they have to get into a different alignment at impact. Maybe this gives a squat - which is not needed especially by a TGM hitter who addresses the ball at impact fix?
Interesting observation. If you look at a lot of these pro swings, they squat and their head drops on the downswing, but then they don't change their level through impact.
To me, the squat look of the legs feels more like a rotary torquing against the ground than an actual vertical squat.