I thought for sure it was a touchback myself.  But listening to David Pollack’s post game podcast, Pollack was adamant that it should have been a touchback (but that it wouldn’t have impacted the outcome of the game nonetheless).  

His partner on the show however, was convinced it WAS a TD.  His logic is that at the very moment Smith crossed the goal line, he did have control and you can see it on the replay.  He made the catch, began to juggle it after a few steps, and then pinned it against his chest with his right hand for a fleeting fraction of a second before bobbling in the end zone. That pinning against his chest is where the refs could not overturn the call on the field. I’d love to see it if anyone can figure out a way to freeze frame the “pinning” so we can post it for the world to see !