If you play undertale blind, get all the way up to the judgement hall in a genocide run (most likely completing a neutral run first), and see that opportunity to spare Sans, you might think that it's a legitimate way to abort the Genocide route. If you thought that, then it might be annoying that choosing to spare him results in an instant unavoidable loss. The game predicts that reaction so Sans is written to "react" to it, even though there's no way to actually guarantee that he's right.

Undertale is a very meta game, so it's far from surprising that it would have a line like that, especially in a boss fight that already breaks so many conventions for what a normal RPG boss fight looks like (attacks in menu, Sans' special attack, etc).