At least as I understand it, the TD can't count until you've become a runner, and you can't become a runner until you complete the process of the catch. If you're going to the ground as you catch the ball, you need to control the ball through the ground (no ground-assisted movement) or you can't complete the process of the catch, and in turn you can't become a runner.
The rule is extremely convoluted, but it isn't the first time someone has been screwed by it. If the rule is called consistently, that's probably all you can ask for.
My issue is whether they could truly overturn the call on the field. Replay evidence has to be 100%, zero doubt, conclusive. I think it's close, but that's not zero doubt. This is from an objective POV, by the way, as I'm not really a Steelers fan (sorry folks).