Those who kill are corpses to begin with. After Situ Panqing cast the Wind Prison spell, he could not escape, so he hid among the corpses of the Imperial Guard.
If a living person had posed as a corpse, they certainly would not have escaped Bai Yan's eyes. Furthermore, with Situ Panqing's spiritual perception, the deception would inevitably have been exposed.
But a corpse would not.
Chapter 126