Izuku immediately nerds out loud right in front of Katsuki, but it's not really to Katsuki. If he weren't there, Izuku would keep going without even noticing. They're not surprising questions. Katsuki remembers what the fox said, but explanations without context, without seeing what she meant, isn't much. He's a Legend, and so is Izuku (of course).

He manages not to joke that it's because Izuku spent most his life quirkless. It's not a quirk thing, so much as he's gathered. Not that the wings on Izuku's back are much of anything. Even flapping, nothing obvious happens. "Don't think you have enough information to nerd mutter your way to an answer, you damn nerd."