Uh-oh.
Wilflare helped me out with Shogun 2, but for some reason the DLC items were added to my inventory ahead of the game itself, so when I attempted to activate the first item, Steam spat an error, telling me it needs the base game; it also mentioned that the content will remain in my inventory until I've activated the game proper.
"No worries", I thought, adding the base game to my library, as well as the remaining DLC items. However, I now find myself without the aforementioned DLC item. It shows up in my account transaction history, but it's nowhere to be seen in my Steam inventory, despite the fact Steam told me I'd find it there.
In short: For fuck's sake, Valve.
Edit: It appeared and I unpacked it, and now the store page says I own the content. Weird. Crisis averted, nonetheless!
Edit: 21GB. Wat.