Well, it is to be expected that he's still recovering from the shock on chapter 255.
I kinda expected Amano to see through his words, though. Well, who am I kidding? She's also probably recovering from the shock (that Manabu realized it was her who wrote on that eraser) when she heard Manabu said that.
Now that both Manabu's and Amano's perceptions of their idols (Mikawa and Manabu, respectively) have been completely shattered before their eyes, they can finally work together to sort things out—to understand each other properly, once and for all.