The NFL studied the tape (sent it to a university or something like that) afterwards and concluded the ball did not go forward. If the ball even goes parallel from where Frank Wycheck throws it, it's still not considered a forward pass.
Now it did kinda look like a forward pass if you just looked at the position of the players bodys. Wycheck did have his arm extended out in front of him, and Dyson reached back behind him to snag the ball. Maybe it was like an optical illusion....but it was not a forward pass....and Randy Moss should have been catching it