What is the point of attacking Poy's body?

I am preparing a battle against the colossus Poy for my two level 6 PCs.

What is the point of attacking Poy's body? It is very specifically immune to all damage, and yet Poy's body has a reaction that triggers for attacking it.

Is this supposed to be some "gotcha" moment that punishes a PC who attempts to attack Poy's body?