They are the same, they are unholy warriors able to resurrect dead entities, they had the Deathcoil and they could heal undead allies or deal damages with it, they are able to sacrify one of their undead allies to get their health (Death pact), they have the unholy aura that enhance attack speed, and they can resurrect an army of ghouls ... like DK's in wow.
That's true that they never affirmed that a hero class would obviously come from WCIII heroes, i used a bad formulation and i apologize, i wanted to give possible reasons for it being an hero class, and i wanted to say that, maybe, it could be cause it was an hero in WCIII.
And, it's written hero class, but it doesn't obviously mean that that it was pointing out the same concept as the death knight, you may be right though.