i saw a video on this. the weapon could be added as a patch then the people who buy the dly wil have they key to unlock it.
they offically announced that itll be a multiplayer weapon in the dlc
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.