The fact that Wei WuXian is still empathizing with Nie MingJue during this scene leads me to believe that Nie MingJue was, perhaps, not a fierce corpse. He was still very much alive even if it was a strange half-life. Jin GuangYao also orders Xue Yang to kill him (They use Baxia! How evil can you get!?) and you can’t kill someone that isn’t alive/is already dead. So CQL Nie MingJue did not die by qi deviation. He died by decapitation.