The ball appears to have been touched in the field of play by the receiving team, so the ball remains alive and in play when it touches the ground in their end zone. When/if the covering official saw the ball touched by the receiving team, yes, he should have signaled for the clock to start. The ball is available for either team to legally recover. The kicking team recovered, so, touchdown is the correct ruling.
If Replay were to get involved - and no doubt they reviewed it - unless they could determine that the ball had not been touched by the receiving team, there would be nothing to change, except to, perhaps, subtract 1 or 2 seconds off the game clock.
If Replay, somehow, was to determine that the ball had not been touched, then, yes, they could have changed the ruling to a touchback. Since that didn't happen, we can surmise that they either clearly saw the ball touch the receiving team player, or could not tell one way or the other, and let the ruling on the field stand.
I correct myself. I was only looking at the high angle view. When I see the close-up view, I don't see the ball touch the receiving team player. Yes, the kicking team player touches it in the field of play, and that is a legal touch that would start the game clock, but the ball would be dead and a touchback the instant it touches the ground in the end zone. Replay could see that and correct the ruling on the field, unless they didn't have this view (don't know why they wouldn't), and also thought that the ball touched the receiving team (like I did on the original view).
Difficult play.