I think he most likely had possession when his elbow touched the ground. However, the call on the field was a fumble. To overturn it, you have to have absolute evidence that it was not a fumble. You can see that the ball was stripped out after the elbow touched the ground. However, the video does not show that he had firm possession of it before it was pulled out. If there was a better camera angle, then maybe the call on the field could be overturned, but you need video that shows the ball in firm possession and not slipping when his elbow touched the ground. That video is not available, at least in what they showed on TV.