Correct call. Bad rule. Although, I think an argument can be made that he established possession and made a football move by diving for the endzone.
Ground can't cause a fumble. But it can cause an incompletion
Just stupid that you can't demonstrate possession of the ball after a catch unless you run with the ball afterwards. If you catch a ball with outstretched arms and tuck it away into your body before hitting the ground, that is possession. Plain and simple.