Actually, that isn't true, or at least isn't entirely true, Stratos...
For example, the FFIV sequel DLC contained a lot of data to be used later, but much of it was unused, revised, or just downright rewritten in the other chapters, and that's only referring to character data, let alone the maps and things.
As far as MLAaDL DLC, which isn't the same as MLAaK :p, the DLC for everything in-game is likely separate from the game, but was almost definitely built before the game's release. There are items, though, with no picture, model, or graphics, that were to make the game easier, essentially just giving you more in-game money. Those could easily have been included, though likely were not.