I wouldn't be so sure. HBS has already taken extensive liberties with established canon. Introducing the ER Medium into the Succession Wars would simply be another modification. We already have an hither-to unknown Periphery power oozing with King Crabs, Highlanders, and Black Knights. Not to mention the incredible Castle Brian cache that we simply blow up. Atlas II anyone? An ER Medium, sure, why not. I don't read that as the Clans are coming.
This is my assumption too. "Sure it never appeared during the Star League era, but they did experiment on making them prior the Amaris Coup/never left the proto type phase/blah blah"