He takes thrre full steps after securing possession, crosses the goal line, and then lets the ball slip out on his way to the ground.
Here are the NFL notes on possession:
Note 1: A player who goes to the ground in the process of attempting to secure possession of a loose ball (with or without contact by an opponent) must maintain control of the ball throughout the process of contacting the ground, whether in the field of play or the end zone. If he loses control of the ball, and the ball touches the ground before he regains control, there is no possession. If he regains control prior to the ball touching the ground, it is a catch, interception, or recovery.
Note 2: If a player goes to the ground out-of-bounds (with or without contact by an opponent) in the process of attempting to secure possession of a loose ball at the sideline, he must maintain complete and continuous control of the ball throughout the process of contacting the ground, or there is no possession.
Note 3: If a player has control of the ball, a slight movement of the ball will not be considered loss of
possession. He must lose control of the ball in order to rule that there has been a loss of possession.
I don't think any of these apply here, as he did not go to the ground "in the process of of attempting to secure possession". He HAD possession, made a "football related move" (ran three steps), and crossed the goal line. Touchdown.
NCAA requires an AIRBORNE receiver, or any receiver going to the ground ON HIS OWN to maintain posession through contact with the ground. This receiver was neither airborne, nor going to the ground on his own.
He had possession, ran three steps and crossed the goal line. Touchdown.
