The Demon Tribe of Red Water State must believe that the Immortal Path is behind the schemes that orchestrated all of this, especially since Li Zhirui released a few devil beasts during the attack, which can testify to this point.
Moreover, the Demon Tribe certainly wouldn't think that the resentful spirits and skeletal remains could have been caused by the mortals of the Great Elder's faction.
If Li Zhirui hadn't seen it with his own eyes and received the legacy, he wouldn't have believed that mortals could possess such formidable methods.
As for the techniques that appear to resemble Demonic Tao divine powers, yet suspect the Immortal Path, it must be that the Demon Tribe believes the Immortal Path did it on purpose to distance themselves.
Otherwise, how can one explain the quiet disappearance of millions of mortals in such a short time?