Would subdomains even help with posts form +? Google has been using +Pages to communicate lately so even if it was an article from Google it still likely would have said plus.google.com.
Yes, because the URL hint would be "plus.google.com" rather than just "google.com." The latter could link to Groups post, a Google+ post, or a page under the official Google site (google.com).
Each represents a rather type of communication which may or may not originate from Google HQ.
