I believe somewhere it says that no demon is allowed in the depths of Antorus, except a few select cases with the Coven and Varimathras I guess. But I believe all the trash mobs are titanic constructs.
That being the case I could imagine that even Archimonde or Kil'jaeden wouldn't be told about it. Sargeras seems like the kinda guy who only see the Legion as a means to an end and they don't need to know his master plans to their full extent and so Aggramar was the true right/left hand of Sargeras all along, for as long as he's been around anyway in his new form after the death of the Pantheon.