Yeah it's a bugged dialogue trigger which should only be available to those who raided the Grove, but for some reason everyone seems to be getting it, usually after the events at Shadowfell. I also got it yesterday as well.
As for why the lines exist; Marcus is the one who convinces Jaheira to spare the player if the Grove has been raided, since Mol is dead. So instead of him swooping in once Isobel is met for the first time, he is instead at the Inn from the very beginning and then goes to wait upstairs for the player to meet him before talking to Isobel. Has some unique dialogue and slightly changes that part of the story.
