This has been answered about 6 billion times, even by Treyarc themselves.
You must still gold every other SMG before diamond camo will unlock on the Peacekeeper. I don't really understand why people would think otherwise. Why would this gun magically get the class diamond camo on it's own? Gold, sure, but not diamond; it's not THAT special. It's still an SMG, and requires the same class camo unlocks as any other gun.
I do understand the confusion about the Peacekeeper impacting the diamond camo for the original guns, though.. and no, the Peacekeeper is not required in any way to get diamond camo for the original SMG class guns.