They're real NPCs, but they're also WoW NPCs. I wouldn't regard them as any of the WoW Death Knight NPCs that existed prior to the Death Knight class.
These NPCs are using Warlock-based abilities, because that is the closest thing that exists. In the case of Illidan, he is a special raid boss, and I wouldn't use that particular version of him as source material for a playable class unless it can be reworked to fit. I mean this is the same as using 4 Horsemen or Arthas as an example as a case for-or-against Death Knights. Just because 4 Horsemen can summon meteors or use Holy magic, Death Knights should be lumped into the same themes? Of course not, those are special named characters with their own identities. Even Baron Rivendare summoned armies of skeletons; an ability that player Death Knights do not have or share.
NPC abilities are a very fickle thing to base any playable class on. If you go down this route, you are making the same fallacies that Teriz has been using saying that Scarlet Monastery and Auchenai Crypts Monks validates the inclusion of the playable Monk class. They have absolutely no correlation to each other.
Any new Demon Hunter class would gain a consistent and concrete theme that fits within their own archetype, with lore explaining so. This would all be derived from the original source, Warcraft 3, as the Monk and Death Knight classes both adhered to. Notice that both of those classes had almost no influence by any existing NPCs existing in the game, they were completely self-contained and drawn inspiration directly through Warcraft 3; even if abilities were not carried over 1:1. If you look at the original Warcraft 3 identity of the Demon Hunter, the only thing it really has in common with Warlocks is the the Demonic transformation. They use no shadow abilities whatsoever. NPCs could be similar to Warlocks, and that would have absolutely no relation to the relationship a Playable DH has with Warlocks.
If you really want to build a case against the Demon Hunter's Identity conflicts, I would at least treat it in the same manner the Death Knight and Monk were added in the game. Both of those classes already faced the same tribulations that the Demon Hunter is being presented with right now, and despite whatever similarities or conflicts that exist, they were able to fit in perfectly.