Hate to rain on the parade here but it's encroachment on B. The snap is not complete until the ball leaves the snapper's hands. To hit the ball out of his hands would be an illegal act.
True, the snap is not COMPLETE until it touches a back or the ground. Doesn't matter. The defense does not have to wait until the snap is COMPLETE, they are free to enter the NZ when the snap BEGINS.
Using your logic, on a punt, the defense can't enter the NZ until the snap reaches the punter (or hits the ground). Obviously, that's not true.
If your logic were to hold, then every team should always be in a shotgun formation, because the defense could do nothing until the ball reached the QB.
By rule, it is LEGAL to knock the ball from the snapper's hands if the defense doesn't cross their LOS until the snap begins. It is d@mn difficult, but it is not impossible, nor illegal.