It isn’t Azshara. Azshara has only been in league with N’zoth and the Void for 10,000 years.
Iridikron struck his “dark bargain” (quoting Alexstrasza) before the Incarnates were imprisoned, which took place 20,000 years before DF.
Azshara and Iridikron wouldn’t even know of each other, most likely – EDIT: Also, they wouldn’t be able to communicate due to Iridikron being imprisoned. Xal’atath, on the other hand, most definitely would have been around to bargain with.
As a person who loves nelf lore, I love Azshara, but that ain’t her as the silhouette. Also, narratively, it doesn’t fit for it to be her unless Blizzard does some gymnastics in established lore.