Denver was not penalized for a false start; they were penalized for illegal motion. The referee's hand signal is the same for both (hands rotating around each other).
The NFL's official play-by-play reads:
(14:53) (Shotgun) 18-P.Manning Aborted. 66-M.Ramirez FUMBLES at DEN 14, recovered by DEN-27-K.Moreno at DEN -8. 27-K.Moreno tackled in End Zone, SAFETY (56-C.Avril). Penalty on DEN-18-P.Manning, Illegal Motion, declined.
A false start is called when "an offensive player
who has assumed a set position charges or moves in such a way as to simulate the start of a play, or if an offensive player who is in motion makes a sudden movement toward the line of scrimmage." (NFL Rules 7.4.2)
Illegal motion (which is not a dead-ball foul) is called when an offensive player is moving forward at the snap or is never set for one second at any point before the snap (NFL Rules 7.4.7).
Peyton Manning was moving towards the line to call an audible when the ball was snapped; but since he was not simulating a snap, it was an illegal motion and not a false start.