If you think it was clearly a fumble, then you must be thinking that the ball has to be in control when he finished his throw. Watch this video and pay attention from the 3 to 4 second mark and the 33 to 34 second mark. Look at the change in position of will's shoulder's between the time you can see the defender's hand and the time you can't. It would take a hell of a move by Will to keep his hand from going forward when turning his shoulders that much.
https://www.youtube.com/watch?v=-bCbF9OXxP8
Will didn't overpower a defensive lineman that got to him before he started him forward movement and managed to will the ball down field 10 yds. He was starting to move forward before he got hit. Just barely, but barely counts under the rule.