The sign on the door has been there for quite some time. I think the meaning is that - unlike LHR for example - it's simply not an Arrivals Lounge. However, I don't see why the OP should have been refused access, assuming of course that they planned on remaining in the lounge until their departing flight. I think that had they planned on popping into the lounge briefly to, say, print something and then to take the Metro into town for a meeting before coming back later then I can kind of see why they were refused access. However, you could equally be arriving into NCL as a domestic passenger on another airline and then departing on BA. So I agree with @
madfish that this does sound very much like someone making up the rules. Imagine if you arrived at LHR from NCL before flying to INV and you had to go through arrivals and back through security. It would be carnage!