It has nothing to do with how you fall. Maintaining possession to the ground is applicable for any case where clear possession (meaning enough time with possession before falling) isn't established. Decleaters, sideline catches, forceouts, etc.
The issue with that particular catch is that he wasn't falling, he was turning. He had possession of the ball with 2 feet down and enough time to turn his body. If you claim the ball was moving in his hands as he was turning, that's fine. The "maintaining possession to the ground" claim is pretty bogus in this case.
These are the cases where I start to hate replay. Nothing in the replay showed anything that the official on the field couldn't or didn't see. The ref just asserted his opinion over the standing call on the field without overwhelming evidence. Replay should never take a correct call and overturn it.