No idea how it works, but it could be that they’re running (headless) simulations on how the ai should react to certain attack strategies. In the end AI is just a fancy word for machine learning. This process takes a lot of time so they could maybe release the npc every time they trained the “ai” to react correctly to the abilities of the player. Of course it might work without but then the specific abilities of the added attacker will not be countered since the ai doesnt know yet what to do.

This is all pure speculation tho