It's not that I'm all that upset about this turn, there are just so many better ways to draw out the story in a way that doesn't make the characters look this dense. Like, we could have Kano accept the confession, then misinterpret Miyu's love as a BFF+ and angst about not being on the same level, that Miyu's affection is a flash in the pan. That would be fine. Or maybe they both confess, but then put it on hold while they sort out what that means, but then the relationship stalls out. That works. Or Kano freaks out and assumes that Miyu is just saying what she wants to hear. (I'm assuming we keep the part where Miyu is just right out in the open, because I think we all agree that's a really refreshing change.)
The point is all of these versions are problems you can't fix by just adding "No, Kano, I'm dead serious. I'm not practicing. I love you." Which she could! She could do that right now! There's no reason she couldn't end this misunderstanding with a flat ten words or less!