He lost control when his right hand comes off the ball, you can tell it wasn't him switching to his left hand because the force of his right hand slipping moved his hand almost all the way to his left elbow.
It was a fumble, it was clear as day, your arm does not move that way if you're switching the ball purposely. After his hand slips off, the ball control is lost, and what you're seeing is him trying to regain possession. Even when he hits the ground, the first hing he does is try to get up and find the ball, but it's already out of bounds.
He fumbled, he knew he fumbled, it's not debatable to anyone who knows anything about sports. Once his right hand comes off, you even see the trajectory of the ball change. That = loss of control
See, that where I think most believe he lost control of the ball, when his right hand comes off of it. I don't agree, it looks to me like he lets go of it with his right hand on purpose, reaching with the ball still in his left hand. It clearly came out, but I still believe it broke the plane before it did.