I think you nailed the contentious point.
I looked at it and thought, he has completed the catch and made a stretch for the TD (a football move)
Whereas referee didn't see the football move, and saw it all as the one action to complete the catch. Given that, they made the right call.
It was very slight, But I do think he made a move for the line. It could've gone another way on another day. One of the tighter calls you'll see at such a crucial time!