But the reason I don't think Matoi is asexual, is because the story is perfectly set up for her to be wrong. The main conflict for Matoi is that she struggles to understand others or herself. From a narrative standpoint, she needs to overcome this conflict by learning about herself, and the people around her. And that works the best, when what she thinks she knows about herself is completely wrong. If she was right from the start, then there wouldn't be much left to learn for her, would it?
I mean, the story explicitly states that Aro and Ace are two different things. They aren't like a pair of shoes you need to wear together. It seems pretty clear that Matoi is at least demiromantic, but probably asexual as well. Sexual attraction and romantic love don't always line up as clearly as some like to think.