Throughout the years, we’ve heard that Demons are beings of Disorder, magically connected to the Twisting Nether, and that the Warlock Voidwalkers were classified as “Demons” for game mechanics, but they weren’t really Demons.
That’s all fine. Except the World-Soul saga decided to add more Demon-Void things.
It started at TWW launch with the addition of Hero Talents. The Warlock Hero Talent “Soul Harvester” gives you a “Demonic Soul” that lives inside you and comes out every once in a while to do stuff.
What is this “Demonic Soul”? A Void Revenant.
Then K’aresh came and it turns out that Observers, another type of Demon that Warlocks have been Summoning since MoP, are also beings of the Void.
(In retrospect is does make a lot of sense, they are masses of eyeballs and tentacles that live to watch and devour things.)
And now with Midnight you have Void Demon Hunters, transforming into Void Metamorphosis.
At this point it just seems like the exceptions are swallowing the rule. There’s so much “Void Demon” stuff that it’s better to just expand the lore definition of a Demon to include Void beings than to maintain the inconsistencies.