That's not how the attack works though: the fob sends A, but the car doesn't see it because of the jamming. The fob then sends B, and the car still doesn't see

That's not how the attack works though: the fob sends A, but the car doesn't see it because of the jamming. The fob then sends B, and the car still doesn't see it. Then the attacker stops jamming the car's receiver, and replays A, so the doors open. B is stored by the attacker to replay later.