Missing Link is weird for DLC, and it kind of rides the line between "standalone expansion" and DLC. It isn't accessed from Deus Ex: Human Revolution (like the other items you mention), you have to load it separately. There's no way to start Missing Link from within Human Revolution, and vice versa.
Usually this means that you don't have to own the base game to play ("standalone expansion") but in this case, you do.
In practical terms, you can install Human Revolution without Missing Link (and I believe vice versa). Most DLC on Steam is not like this - once you own the DLC, you get no choice as to whether or not it is installed with the base game.