Airbourne player charges into backward moving defender

[MiamiWadeCounty]

Lets say: B1, in a LGP, slowly moves backward in anticipation of A1's move. A1 jumps forward and toward B1. B1 continues to slowly move backward until airborne player A1 charges into B1.

According to the rules, this should be a foul on B1 because B1 obtained his legal position (the position at the time of contact) after A1 has left the floor, 4-23-4.b. A defender who moves backward after the opponent has jumped towards him is obtaining a new position after that opponent has left the floor. Isn't that a violation of rule 4-23-4.b?