Nobody suggests he fumbled.
He didn't.
Well if that white object in the fuzzy picture was the football, it was a fumble. But as is shown later that white object is the wristband on the arm holding on to the football. So we have concluded the wristband did break the plane of the endzone. But it was a real close call and you can't fault the ref. In reality, the ref gave Pittsburg the TD because Ben snuck the ball over the goal line after he was down. There is no way the ref could have determined if the ball broke the plane from 20 yards away.