This entire thread stems from a huge lack of understanding about the words used.
You're able to conquer something, and not retain ownership of it in the long-term. In the Alliance version of the Siege of Orgrimmar, the Alliance do conquer Orgrimmar. They rip through the defenses, and kill all in their path. For that week, in that instance, they have conquered Orgrimmar.
In terms of Lore and Story, they did conquer it while the Horde liberated it. Just look back through history, France has conquered the U.K, Germany has conquered France and the U.K. has conquered Germany. Yet here we all are, still in our own nations ...
Conquer =/= Ownership