If you go into the World Editor and place a Death Knight, it uses Arthas’ model and the original Death Knight voice. Notably, if you edit the “Death Knight” properties in the editor the hero is using the [Udea] skin. Arthas’ skin is [Uder]. While the female Death Knight’s skin is [Udef].
So there’s actually three Death Knight entities that makes me wonder if they just haven’t finished a non-Arthas male DK.