The ball touched the ground, but Dez was a runner down by contact. It was incorrectly ruled that Dez hadn't established himself as a runner, so he had to maintain possession of the ball through contacting the ground -- incomplete.
If Dez had been ruled to have established himself as a runner, then it's Dallas ball at the 1. The ground can't cause a fumble.
What anybody who supports the overturn has to prove (whether they realize it or not) is that Dez didn't establish himself as a runner. That involves proving he didn't reach for the goal line. The official explanation is that there was a reach, but that it wasn't obvious enough. Reasons being, Dez didn't use two hands, or didn't extend the ball for the goal line. Even though there's no rule about using two hands, and extending the arm wouldn't have put the ball any nearer the goal line.
You'll hear a lot of noise unrelated to the actual play, but that's what the guy who made the call says it came down to. If his opinion means anything.