To start off with, I'm not an Indy fan and am indifferent on SF...I don't really care much about how the game came out. (just a disclosure)
I just watched it in high definition, rather than the crappy twitter video....the play before was brutal cause Hines dropped the ball and he was wide open on a 3 and 7. Indy would have definitely scored a TD, but instead had to punt.
The returner was out of position and muffed the punt. It was incidental contact because he never had control of the ball, but because he touched the ball, it put his team at risk to lose possession. At that very point, it could have been returned by the Colts near or in the end zone for points!
He only gained control of the ball in the end zone. That's why it was a touchback. He didn't have possession of the ball until he and the ball were both in the end zone, and it was a live ball that anyone could have gotten prior to that.
As for it not being a safety, he was in the back of the endzone, but at the tail end of the play, he was dragged backwards and pulled down while tackled while he maintained possession. Forward momentum rule is what made it a touchback because he never had possession prior to that.
What ifs...
Had he kicked the ball out of the back of the endzone himself, I think the contact would have been reviewed and perhaps a safety would have been awarded...I really don't know because that counters the logic of it being a live ball since a safety typically requires possession and the offensive player's end zone. It would definitely be worthy of some kind of penalty if he were kicking the ball so the other team couldn't score a touchdown. Someone might know the rule book on that play if it was ever documented.