OK, I finally found it. The "second act" thing was a creation after the Lance Moore 2 pt conversion in the SB a few years back. It is not technically in the rule book but Pereira insisted on it being ruled that way during his tenure and it hasn't been changed since.
So I thought the league had said the second motion does not negate the need for maintaining possession but they actually said it does. Whether the player was down or not is what determined, in the field of play, whether it's a fumble or down by contact first.
Just for reference, here are two plays which Pereira says should have been ruled a score according to the rule.
So the only thing that really needs to be addressed at this point is what a player already in the endzone can do to perform a second act. Moore, in his, was actually extending the ball across the goal line before any major contact occurred. Once the contact occurred, he lost the ball. If there's no similar way for players to establish that they had possession before contact if they're already in the endzone, I think it'll continue to be imbalanced and called incorrectly at times.
For now, though, it does appear they called the Alexander TD in accordance with precedent though not technically in accordance with the rule book.