Yeah, it's pretty much this. Though there's also the general rule of thumb for trying to destroy something significant, that no matter how much you can get rid of, you'll miss something.
But there's also been a string of organizations, including Foundation precursors and other secret societies (including proto-Serpent's Hand orgs as well) using anomalous means to suppress information and items that did survive. For the past, I dunno, literally thousands of years. There's still the risk that information might get outed, of course, but it hasn't happened yet.