Manning didn’t fumble it. The football was hiked over his head.
For the record and the first play from the line of scrimmage of the game. The Broncos center Ramirez is “awarded” the fumble and not Manning.
From the official game stats:
“(14:53) (Shotgun) 18-P.Manning Aborted. 66-M.Ramirez FUMBLES at DEN 14,...”