I think that GURPS simply changed a few things (creating undead included) to make the sytem run well as an RPG. To allow PCs to create their own undead, or at least to incorporate it in a GURPS game, I think they changed things to allow any mage to do this. In myth it is not as simple. The Watcher (Bah'al) used the Dream of Unlife to turn the fallen warriors at Avons Grove into Thrall. The Dream of Unlife seems to be the only way to create undead in TFL. Yet Soulblighter has his own Undead forces, as you say he's not much of a mage.
So The Dream of Unlife is generally the only method of creating the undead, and Bah'al was the one with the dream of unlife.
