The operative term being "with possession" when receiving a catch and falling down a player is not "with possession" until they finish falling down and still holding onto the ball. If he had two steps and THEN started falling (like from getting hit) it would have been a completion, but he was falling before the ball touched him that meant the fall rule applied, and the fall rule says you have to keep holding onto the ball until you fall.
That means a running back who crosses the goal line can theorically fumble after scoring. I saw two steps then the knee hit then the ball came out.