It doesn’t one-shot riders that are facing the dragon:
What happens here is not piercing damage. It’s just an artifact of how the fireball is processed. It hits the first square (empty), then hits the second square (knocks the rider off its horse and back to the third square), then hits the third square (hits the rider again, knocking off its shield).
The same thing happens with firepigs, but not with player fireballs, for some reason.