It was bobbled against the ground... but he had crossed the plane. I guess they decided that the pass-catching completion rules dominated the passing the goal-line plane rule.
Even the announcers were surprised. Romo said, “Oh... that’s what they’re discussing on the review”, after first saying “Touchdown!”.
It used to be the ground couldn’t cause a fumble. And I didn’t see any ball movement before he landed in the end zone.