The fact that the structure is there on disk, the entire scenery structure. All the guns, and armor thats planned. All the placeholders for the levels. Means that the DLC was almost fully developed at launch. How hard do you think it would be to turn off the enemies for them? What is the game if not scenery. Thats the thing that takes longest to develop. Enemies are just bigger versions of themselves. The AI is already been programmed. All that is needed is to make a short story structure, and put in the sounds and cinematic.