I don't think anybody here expects a formal reprint with official errata to change some Spirit creature subtypes to/from "undead". I think it's more a thematic question of "What does it mean to be 'undead' in the Mage Wars universe?"
Illusions, elemental spirits (like Whirling Spirit), the Imperial Stalker, etc, would seem to be non-living incorporeal (or not actually incorporeal, in the case of Academy illusions) creatures that were never Living. Is "undead" simply a non-living creature that was at one previous time living? If so, then Grey Wraith would likely qualify, as do the other new Spirit "undead" creatures. Or does "undead" require being a corporeal construct of the remains of formerly living creatures, like a zombie, mummy or skeleton, in which case the Grey Wraith would not qualify (it can effectively walk through walls after all), although then it would stand to reason that the new nonliving Spirits ought not to be "undead" either - e.g. a creature would be disqualified from being "undead" if it is incorporeal.