[Yeah, there's still no shock or confusion to be had here, even the joking kind. Papyrus nods with a relieved and satisfied smile at that agreement, instead. It's the relief that might be the giveaway, to Sans's eyes - relief that he's not alone, in puzzling all of this out.

Sure, Papyrus himself had maybe backslid somewhat into uncertainty and nerves and following others' leads, those first most disoriented days. And, well, maybe he's still not seeking all that much responsibility just yet, more tired deep in his bones than he'd care to admit. More relieved that the people who'd been looking up to him... aren't here, needing him, than he'd ever care to admit either. But... He still accepted that responsibility, and needs to try to get back to them. That will surely be easier with both of them working together, even with the terrible antique technology!]

I doubt they're too unfamiliar... [He muses aloud instead of saying any of that, with a glance off towards the man who mentioned ICBMs.] Who fights a war without knowing, where their opponents are from? If they don't know... Can you really call that a war? I call that an ambush.