He raises an eyebrow because he hasn't explained to Tony the full situation - he'll have to, someday, just in case it happens to him because the whole problem was that he had kept secrets from Tony in the first place. That's why he's here, checking up, because he hadn't gotten to, in person, for years. But he won't defend himself on this part, he'll just say, "And that's how I know it's the wrong move," as he gently rests a hand on the back of Tony's head, to keep him there, assure him that there's a spot for him on Steve's shoulder.

Why Tony continues looking for something to argue about is beyond him. He'd rather they just work past this, that they recognize that they're going to be slightly different versions of the men they know, that they each come with similar problems but also very familiar ones as well, unfortunately. "How about sleep?" he asks. "Can't find anyone who's slept longer than me and isn't dead, and I can tell you that you need some."