Well, if anyone has never been particularly encouraged by Steve Rogers wanting to fight something, it's Bucky Barnes, so Steve isn't necessarily put out. He still thinks they'll be okay, though, if they do have to fight. Especially if they've got some backup.

"Yeah," Steve agrees, because it must be what happened to him. Not that he has any idea what's going to happen to him after the fact, but that's something to worry about later. "I mean... sure, you did kinda shoot her, but I think she'd understand," Steve points out. "I did."

As if his penchant for forgiveness is at all shared by Natasha. (It's not.) But still, she does understand. He knows she does. And he just assumes she's his Natasha, like he assumes this is his Bucky.