Distracted as he was, the cultist hadn't been expecting an attack now that he had the boy where he wanted him. So the attack works, and he's thrown into the nearest wall as he shapeshifts his arm into something thinner in a rush to try to get it out of her jaws. The black rope is still in his hand.

Once they see their friend being attacked the two cultists that had been shifting heir skin to stand the fire throw themselves at Zuko, black knives in their hands. Now they know how dangerous he can be.