It’s a good point, but that implies there was some level of doubt in their minds. But I just don’t see how there could have been. Nobody was around him, he was airborn, and then the ball popped out when he slammed back on the ground. Your suggestion is a good one if there is any doubt. I’m just wondering how they had any doubt.
I don't know, watching it live I couldn't tell if he held onto it all the way to the ground or if it was starting to come up. I thought the refs did the right thing for once. I said out loud "Oooo, that might be a fumble . . ."
If you watch the super slow replay, the ball does actually move off his forearm a little about 3-4 inches before he hits the ground.
Generally though, you can tell by a player's body language afterward whether they fumbled it or not. If they just get up like what's the big deal? The way Tua did, it's probably not a fumble. When they are immediately scrambling for the ball, they think they lost it and it most likely is a fumble.