It buzzes through him after it's said. That giddiness in one's body to be so bold, and he can keep this up! Shame being told to wait in the back of the car while he displays himself as bold as could be. Inigo's laughter an encouragement, a reason to go on. But also, to Inigo's actual remark-

"Why wait? I'm more than a mission! I'll take on anyone who comes at us!"

Hear that, Inigo!? He will! (And that means you too, Reno!)