"Correction to Yarvin: Satan is the father of lies, “the Cathedral” is a
distributed lie-creation network. Ergo, it is not a cathedral, a
beautiful thing, it is Pandemonium—literally, “place of all demons”; and
it creates literal pandemonium, chaos, as well."
Let [a] refer to that which leftitsts believe cathedrals to be and represent.
Let [b] refer to that which leftists have in fact built.
It's also a reference to Eric Raymond's book, The Cathedral and the Bazaar. Raymond is basically a leftist and fully believes in leftist mythology about cathedrals. The leftists, Raymond, and many alleged outer rightists, believe Fascism is a bazaar-like phenomenon. (The exceptions think it's about the Gyews.)
This is why Yarvin's term is more on-point than not.
That said, it is something of a joke or troll. Ironically it lampoons Yarvin's own beliefs, which makes it even better.
Cathedrals themselves are of course beautiful buildings.
I personally believe them to be an attempt to stave off Sophism. With a typical cathedral planned to take 100 years or more to build, it is one of the very few virtue signals that in fact signal virtue.
From the fall of Rome to 1100, Christianity was basically healthy. (At least sort of healthy.) Most Cathedrals were started around the time you would expect the infection from Toledo to reach the place. E.g. Notre-Dame late 1100s, Florence 1200s, Russia, 1500s. If they were caused by Sophism, they would suck. They don't suck, so by process of elimination they're a reaction. Checksum: modern Sophists hate cathedrals and want them destroyed.
Unfortunately, it turns out Christianity was never healthy enough.
P.S. Many of the Gyew-obsessed wish to rename Cathedral to Synagogue, exactly the same way a bunch of pitiful, lame leftists made /r/therightcantmeme in reaction to left-can't-meme. It's the difference between having your own culture and having to copy someone else's. See also: there's this je-ne-sais quoi about Europeans copying DNC talking points word for word. In English.