Iris has a thing for older guys. In this case, much, much, much older guys.

+ bossy Iris and Ardyn is just delighted to go with it
++ Ardyn totally spoils her and honestly treats her like a princess (he doesn't need to hurt her to piss of the bros, it's agitating enough for them that Iris was the pursuer in this case)
+++++ they keep doing it during the WOR after the truth is out but it turns into incredibly aggressive hatesex that Iris won't admit to on her part, and fun BDSM subbing for Ardyn on his part