It's that everything good created by Muggles--from buildings to art to literature--has to have actually been made by wizards.
The first time I saw it, I thought, "Oh, that's interesting." And the thing is, for that fic, it worked. It wasn't an offhanded, smug, "Wizards are so much better OMG!" comment.
But I swear to all that it holy, the next time I see "Shakespeare/Tolkien/da Vinci was a wizard, didn't you know?" I will scream.