Hmm, I know epistemology can be hard to fathom (and frankly is not my thing, my thing being philosophy of language), but I thought my statement was one of the basic definitions. I would be interested if you can point me to work that considers a statement like "I know that Chicago is the capital of Illinois" (it's Springfield) tenable. I was taught that this was an incorrect usage of know.

Although he's probably right that Rumsfeld should've mentioned it, because there's something like unknown knowns for groups with a coordination problem (like the 17-branched US intelligence community), where some people know something but that knowledge doesn't propagate to the rest of the group.

The article is not referring to subconscious drives as knowledge. It refers to cultural assumptions that few people question as "unknown knowns". Many of these assumptions are the kind of things that PG refers to in "what you can't say".