I agree with you - he started the somersault when he left the ground at the 4 yard line. It wasn't something he started after the ball crossed the plane. The call was probably wrong, but at this point its water under the bridge.
... It's hard to tell when the ball crosses the front of the goal line, as he stretches the ball out, and then pulls it back to start his flip.
The thing is, that's not how it works. You can't leave the ground, to dive, and suddenly, in mid-air, change your mind and decide to turn it into a flip - it's too late by that point. The entire dive was intended to be a flip from the moment he left the ground, and you can tell that because, as pointed out in the original post, but the time he stretched the ball out for the goal line, his feet were already higher than his head, and rising.
Edit - fixed typo
Last Edited: 9/30/2012 1:11:36 PM by L.C.