Maybe he's thinking about how to approach it. Or he's trying to gauge how you feel first.

But if he told Akira you're unavailable, then he must have thought his friend also wants to court you and would be competition. [Competing for someone who doesn't even know she's the trophy.] If he didn't agree, do you think he would have left, or found you so "tempting"?

Regardless, what is most important here is how you feel, and no matter how Denji or Akira see you, neither of them should dictate your potential swains for you.