I noticed the same thing. I tweeted Dan (@OneOfSwords) and he replied back with "Probably because it's additional content and not part of the core game - it's a bonus weapon".
He did not say anything about them fixing it or adding it. Like you, I was hoping to complete the "SMG Kills" challenge because I enjoy using the Peacekeeper.
I will update if I hear anything else, I also tweeted the same question to Treyarch, Call of Duty, and David Vonderharr.
I updated my post in the Technical Support forum but thought I would update here as well:
- Still have heard nothing from those I tweeted except Dan. I asked David Vonderhaar and Activision Support (@ATVIAssist) again but got no reply. I assume Dan's response is the official reply. I am fine with that outcome but it sure would have been nice if it counted towards the "SMG Kills" challenge. I may keep tweeting both David and Activision Support every other day in hopes of a reply.