Highly doubt that you need it to get diamond. That would mean they'd have to reset diamond for the players that already have it on those categories, which I do not believe they'd do.
So assuming it's in MP, how will it work for players who don't get the DLC? They can't equip it in CaC but they can pick it up in-game if someone else drops it or what?
According to Vahn's latest tweet you will be able to pick up the Peacekeeper off of the ground, use it in Sharpshooter and Custom games even if you don't buy the DLC. So my guess is that it was built into the disk all along and is just getting unlocked. You will also not lose your Diamond SMG camo if you already have it and can get the Diamond camo for the Peacekeeper just by leveling it up to Gold.