He’s classified as undead. He’s undead during the battle. His body was literally torn back into place and Mannoroth was forced to reconstitution by Gul’dan. That is a literal undead demon.
The ritual to bring back a mortal or a demon to their old corpse using necromancy or fel remains the same. Just as raising Calia into undeath via the Light remains the same. Undeath is undeath no matter the magic used to fuel it. The ritual is what’s important and an undead demon is merely a demon who has been reconstituted into “life” via necromancy forcing his/her soul back into their bodies instead of the natural cycle of having their souls return to the Twisting Nether to reform a new body.
That goes against precedence in the lore. We know it’s not the magic power it’s the ritual and outcome desired. In the case of a Man’ari Death Knight the ritual of forcing their souls back into their bodies to achieve a Death Knight soldier is what’s the endgoal. Just like it was with Calia via the Light, Mannoroth via the Fel/Shadow, and as with now playable Death Knight Man’ari. They are another version of Death Knight using of different material. In this case, a Man’ari corpse.