I didn’t exist not because it didn’t exist in game… it merely wasn’t written. There’s a difference between retcon and developing a concept. You’re assuming because the void wasn’t expanded on that it was originally based in the Twisting Nether. The absence of lore does not define the lore. Void was lumped with fel demonology because the void lore hadn’t been developed and the voidwalker had to have a faction tag.
Because the universe hadn’t been fleshed out, it was assumed that void/shadow came from the Twisting Nether. You have to remember, WoW lore is based on WC lore, and WC lore was not that detailed. WoW lore was also very under developed because no one ever expected the game, and the genre, to require it as mmorpg’s were uncharted territory then.