It could be that we just haven't discovered them yet. Who the hell knows what they have hidden in this game... the Black Spindle would probably still be undiscovered if the door to the mission wasn't very noticeably in motion when you get into that last room with the tomb husks. I highly doubt it, but it's possible. And yeah, it's pretty shitty that we don't have access to all of the advertised exotics yet.