DLC doesn't usually introduce new code to a game, that tends to be reserved for full-on title updates. DLC is more like if Sonic and Knuckles could also support special "new level" cartridges, that didn't do anything on their own, but when inserted on top of S&K provided new levels for the player, but didn't themselves contain any program code.
Every game I've worked on (360-era) with DLC has needed supporting code to be distributed in an update first. Everyone gets the update, then the DLC is either just an unlock for what's already there (but hidden), or contains some supporting data.
Right, but that's not absolutely universal. Harmonix does pretty well continuing to release DLC without needing an additional title update in some time now. My point was more that the DLC packages themselves don't often, if ever, come with actual code included, and this is usually for platform security reasons (Sony and Microsoft are both quite controlling about how executable data is managed and how it is stored). Perhaps maybe some scripts for the internal engine, depending on the game.
