It hasn't gotten anything to do with graphics. It's just that they don't want to exaggerate anymore, and that sucks, it one of the reasons that made the Mario & Luigi games so comical.
Bowser's sprite is the one from DT as far as I can tell. Which does fit more than the PJ one which isn't much different besides being at a slightly more horizontal angle like the rest of PJ's sprites, which wouldn't work as well at the angle this game is in.
I just thought of something. The video just goes into the game, despite there being other files. Files are confirmed through what was talked about, but this shows that either (A)you go directly into the game the first time you play it, or (B)the producers are hiding how many save files there are (Is it 2 or 3).
I don't like the extra part on the battle theme tbh, I would rather that they kept it the same.
The first 10 minutes are rather uninteresting, it's just Mario & Luigi: Superstar Saga with different graphics. I'm more interested in the gameplay changes.