That much was true. He's intimidated her, but he hadn't laid a hand on her. For Briareos that wasn't exactly nothing.
But what would happen tomorrow when this wore off and he could do whatever he liked? What would happen when he decided he was angry with Aphrodite and Eros playing their games and decided to lash out at the object of that infatuation?
Kymopoleia looked at him seriously, thinking about it, and then said, "Swear upon Styx that you mean me no harm." It was a major question to ask, but she needed to know if he would do it. Maybe now, under this thrall, was the best time to ask it. "Swear it and mean it, and it will still hold true when you stop loving me."