Another possible reason Thordan and the Heavens' Ward don't show signs of tempering could be that the ritual the Ascians taught them didn't include the "tempering clause," so to speak. The main purpose of tempering is, arguably, to make the worshippers summon the Primal again and again to bleed the land of aether; since the Ascians knew Thordan and the Knights would use (and abuse) that power, there was no need to temper them.

Or perhaps it's got something to do with the "summoning into themselves" bit... maybe the Echo isn't needed to resist or prevent tempering if the Primal (though the Knights Twelve are more like demi-Primals) is summoned into a real person? /shrug