Now that you've read this blog post, the next time you hear someone unhesitatingly repeating a meme you think is silly or false, you'll think, "Cached thoughts." My belief is now there in your mind, waiting to complete the pattern. But is it true? Don't let your mind complete the pattern! Think!

So, there are people who disagree with what you posted, and may be inclined to argue about it. That, combined with the idea shared in the Paul Graham quote in this very thread (about politics frequently being used as a form of identity) leads to defensiveness, leads to rationalization, and leads to stupidity.

So, in order to avoid stupid arguments, people would prefer fewer posts like your quote on LW.

So there wasn't anything especially related to explicit rationality in the quote. Or rather, it wasn't extraordinarily rational enough to make up for its extraordinary politicalness.