Well actually I've recently noticed that your Sith Warrior is addressed as "Lord" in those chapter cards/intros of whatever you call them. So I think its possible he/she may be a Lord, but can easily be RPed as a Darth.
Now about the actual topic, despite what happened in Beta, the SW is a Darth. He usurped a former Darth and also, unlike Lord Scourge, is very formally recognised by the Council. For that to happen, he would be named a Darth so the Councillors wouldn't lose face inviting them to their tea parties.