I think the call was complicated because both players were grabbing the ball which somehow nullified the 3 steps taken in the end zone. Then he gets possession out of bounds, takes 2 more steps, falls and loses the ball.
Unbelievable, should have been ruled a catch.
Simultaneous possession goes to the receiver so the 3 steps in the end zone = touchdown. Falling down 2 steps out of bounds is irrelevant.
This was a bad call, period.