Systems being managed by a site has nothing to do with the DP or it belonging to the 2012 site.
To manage a system in 2012, it must have the 2012 agent on it *and* it must be assigned to the 2012 site. Neither of these have anything to do with the DP (although the 2012 client bits will come from the DP if the boundaries are set correctly).
Thus, you're missing actually deploying the client agent:
https://technet.microsoft.com/en-us/library/gg712283.aspx?f=255&MSPPError=-2147217396
Jason | http://blog.configmgrftw.com | @jasonsandys
