I imagine it’s just bugged, because I’ve tried spawning in a whole bunch more, of both Thag and the Khari Remnant skeletons, killing them all and then chopping them up so all their loot ends in a bag. Lots of the weapons that Thag drops, hammer, scythe and mace, but none of the Pristine Khari weapons at all.
Edit: Hell, I’ve also gone through the dungeon a couple of times with no drops of the Pristine Khari weapons either. So again, I figure the drop rate is just “stupidly” low.