"My undead servants have never been alive" ???
If I recall correctly, necromancers make flesh puppets, they don’t raise the dead. .That is, the animating force of their minions aren’t any sort of spirit or soul, but the same sort of magical energy that animates creatures such as elementals. To put in in game turns, his servants are immune to Zhaitan’s corruption the same way something like a sculpture would be immune to Primordius’ or Kraalkatorik’s, because a sculpture is just rock, with no animating force, and the Dragons seem incapable of actually creating anything.
Necromancers essentially make golems out of bits of the dead, they are only alive as long as magic holds them together. That is my interpretation of it anyway.
Practitioners of the dark arts, necromancers summon the dead, wield the power of lost souls, and literally suck the life force from the enemy. Necromancers feed on life force, which they can use to bring allies back from the brink or cheat death itself.
I’d interpret this as necromancers utilize drained “life force” to make magical constructs using organic materials, or use magic to create bodies for summoned/contracted “lost souls” to inhabit.