Sans raises an eyebrow at the "other universe thing", not because he can't imagine it's an option but because it was suggested so casually. Sans knows more about the shifting timelines and circumstances of his own world than most, and he's never been keen on sharing that. It raises some questions as to this guy's circumstances. The cause of his blindness being back in that maybe other universe does, too, though it's not like he's a doctor. Sure, when he thinks blindness he thinks (okay, first he thinks of monsters who just naturally can't see and use magic or some other sense to get around, but after that--) of eye damage, but who knows? A doctor. Which Sans is not. That's the point here.

"Is universe-hopping a thing where you're from?" Sans asks with obvious curiosity and no suggestion he's dismissing it offhand. "Most people'd go with something more... Occam's razor-y."