Even if the sarcasm doesn't quite come through, it seems clear the other man hasn't challenged that particular assumption yet. Sephiroth has little practice relating to others, so maybe he's forgotten how much his own mind balked at the idea of being on another world, even in the face of so much unfamiliarity. Even if this were some version of Earth, he isn't well qualified to recognize it.

"...does it seem like Earth? I had some difficulty locating any maps before."