I generally really dislike the "the character was just being retarded" argument. Saying she didn't use ACoC because she underestimated them is writing the story for Oda, he hasn't made it clear that was the case at all so that's just speculation. Also, if someone is on your stomach with a sword in your chest, clearly charging up an attack that'll destroy your internal organs, I'd kinda doubt she'd just intentionally not use ACoC because, eh, they're weak I don't need to.