NE vs DENVER, backward pass review

With Denver up 7-0, Manning threw a pass to the left that was reviewed and called a backward pass ( which it was) my question is watching the live play and the replay, it looks and sounds like the Umpire and Flank Official signaled incomplete and blew their whistles before NE picked up the ball. I'm a seasoned High School Official, in HS this would have been an Inadvertent Whistle, why did NE get the ball? With NFL replay, is that something that cannot be ruled on? Thanks