I still say it is illegal and here is my rule coverage.
The snapper is not a runner so he is not like anyone else with the ball to be swiped at. The scrimmage has to start with a legal snap. If B interferes with that it is a disconcerting act and should be penalized, if there is contact with that act it should be considered UNR.
B has to start this motion prior to the snap to have any chance of slapping the ball.
9-5-1d. Using disconcerting acts or words prior to the snap in an attempt to
interfere with A’s signals or movements
Read more:
CoachHuey.com - Interfering with the Snap