The chances that it's Settra is obviously close to zero for this one, however that doesn't mean he can't be in another Stormvault.
GW could even spin a nice narrative around it. Something like:
When Sigmar discovered the realms and assembled "his" pantheon of gods he found not just Nagash but also Settra. Obviously the super powered Nagash was going to be the god for the undead realms, however we all know Settras personality and so does Sigmar. Unyielding and full of hate for Nagash he would never stop waging war against the deathgod which all in all would've been a bad thing for the mortal realms. So the "best" solution for Sigmar was to lock and hide Settra away forever. Oh the irony of a Tomb King being locked away in what's basically a magical tomb ,., maybe it's even literally his tomb and Sigmar did it while Settra was asleep. Locate it in the great deserts of Hysh, the realm of light (remember that Settra was wielding a blade blessed by their sun god Ptra), far away from Nagashs influence and voíla.