The permissions data which the redeem code gives you is basically DLC. However, the armor it unlocks can still be bought in game. Case in point: Recon. Once you hit commander, you can buy it for 100k. Not that anyone would really want to throw down that much cR for that shoddy helmet...

Bungie's cache system doesn't allow for DLC game data (new armor, a new weapon, etc) which isn't already present in the retail data (read: what came on your disc). Now, a DLC map could include a new weapon (IIRC, they did this with the golf club in that last forge map in H3), but it'd only be placeable in Forge. A game variant wouldn't be able to select it (since the game data wouldn't exist in the mainmenu cache/map file)