Tadano knows it's him. And he's intentionally being 'dense', precisely because he's already had those thoughts that Rumiko expressed. He doesn't know how he'll respond and he doesn't want things to get awkward between them three. So he pretends being an idiot and buys himself more time to think things through.
This is all within his character. He reads the atmosphere and doesn't rock the boat, so to speak. Add his deep inferiority issues and it's not out of character for him to try to keep things comfy for as long as he can.