Well, besides using Shadow Magic (which is an assumption, mainly. If you're interested, the only strong argument against all shadow magic being the Void are DKs, and there's a possible argument there too), the TCG (not canon? I don't know), and the RPG ("not" canon) - which basically eliminates all possible lore for magic in the WoW universe - we have Void Walkers.
https://twitter.com/DaveKosak/status/442062392281665536
http://us.battle.net/wow/en/forum/topic/12405830492
Which are not demons and are intrinsically related to the void and shadow magic. Then there's also the thematics of affliction, namely focusing spells with vampiric-type mechanics or soul/undead themes (ie soul drain) which have an established in lore relationship to the Void.
The argument for relies mainly on circumstance and assumptions, and the only argument against it is just an argument of ignorance mainly. Really, there's shit for Void/Shadow lore in the first place.
This is of course in spite of the fact that that original post was sarcastic.