Board Thread:General Discussion/@comment-27524978-20160210030742/@comment-27701762-20160401014404

Uh, that blue attack line goes away in a standard Genocide run as well. If it's your first time playing that route, he says that you're going to have a bad time if you continue the way you are.

Even the change in the line is perfectly consistent with Sans having only a vague sensation of what happened in another timeline, just as Toriel remembers your preference for cinnamon or butterscotch. He simply knows that that vague sensation isn't a random occurrence.