And much more knowledgeable folks find the following in the NFL Case Book that says otherwise.
A.R. 8.12 GOING TO THE GROUND—COMPLETE PASS First-and-10-on B25. A1 throws a pass to A2 who controls the ball and gets one foot down before he is contacted by B1. He goes to the ground as a result of the contact, gets his second foot down, and with the ball in his right arm, he braces himself at the three-yard line with his left hand and simultaneously lunges forward toward the goal line. When he lands in the end zone, the ball comes out. Ruling: Touchdown Team A. Kickoff A35. The pass is complete. When the receiver hits the ground in the end zone, it is the result of lunging forward after bracing himself at the three-yard line and is not part of the process of the catch. Since the ball crossed the goal line, it is a touchdown. If the ball is short of the goal line, it is a catch, and A2 is down by contact.
There is the rule applied where parts B and C were completed after the going to the ground began. That is from the official NFL Case Book and clearly says that A-C can all occur after a player falls as long as all 3 are complete before he lands.