His knee AND his elbow are down and he has possession of the ball across the goal line. Even before that, he catches the ball, brings it in to his waist and then reaches to put the ball across the plane of the end zone. THat is a football move and two body parts on the ground with possession in the end zone. IN every other universe that is called a TD except, i guess, in the bizarro world of the NFL - No F*cking Logic
Horrific call.,
It's not the same as being "down" when tackled. A receiver can be tumbling 10 yards out of bounds and if the ball comes out at any point before his body comes to rest, they will call it incomplete.