I assume the ball would be placed at the spot where he became down, and if the ball is over the goal line when that occurs, then he should be ruled in the end zone and it's a touchback. If that is the rule, then I think the ball should have been marked at the 20.
On the other hand, the entire ball has to be over the goal line for it to be a safety in your own end zone, so does that concept come into play? I mean, would the whole ball have to be over or behind the goal line for it to be a touchback?
I'm still mystified as to why it was spotted at the 1, since it looks like he didn't catch it at the 1, and he wasn't down at the 1.