What are you even on about?
https://wowwiki.fandom.com/wiki/Magic_schools_(lore)
Necromancy[9] — Study of magic involving the dead. Not used by mages, but basics taught to understand their enemies.
NPCs can do anything, players have to abide both gameplay and lore.
So if DK uses necromancy magic he is in fact necromancer.
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No need to speak for them, they spoke it themselves.
From Jeff interview. And it is a fact, that blizzard considered necromancer, also it is a fact that they did incorporated it into DK.
And read posts carefully as you are obviously grasping at straws, NPC can do anything, playable characters can't. It has to be somewhat unique, abide lore and have sensible rotation. NPC won't give a shit his rotation is 2 button borefest.