If the ball touches the ground, in any capacity, and the ball subsequently comes loose then it's incomplete. Period. In this case, right after the moment where the ball may or may not have touched the ground the ball came loose in his grip, clear as day, before he regained control. The reason the play stood is because there was no conclusive evidence to say that the ball did not touch the ground before it came loose.
But, no one is using that challenge as an excuse if Cam didn't get curb stomped by Von Miller a play or two later, are they?
OK. I think that is the way a lot of people took it. I certainly did. I have heard from others who felt that way. But, you have been around a long time so I will defer to your judgement. Perhaps I got it wrong.