Call? or no call?

[bbcoach7]

Both teams are in double bonus. Team A has just scored to take a 1 point lead with 7 seconds left on the clock. B1 grabs ball out of the net and steps out of bounds to throw in, and is immediately being guarded at the end line by A1. B2 runs up to the baseline and sets a legal screen along the endline. B1 runs the baseline toward B2's screen, and A1 runs straight into B2's screen knocking B2 over, and it's clearly not a "flop." A1 had 3 steps before impact. Is this an defensive foul?

Just to be clear, this is hypothetical, I've not seen it done, but we have practiced it a couple times in case we ever have an opportunity to try it.

>>edited to change my question to is this a defensive foul?<<