Since God's Rhema is His hypostasis, it doesn't include anything created whatsoever; so that's why Satan is a lie and the father of it. Truth is the unveiled reality lying at the base of, and agreeing with, an appearance; the manifested or the veritable essence of all matter. God's Logos is truth, demonstrating that Satan cannot be, have, know, or say the truth; because there's no truth in him.
The serpent's mindset was only noumenological, so it isn't God's Rhema. That's exactly why it's another rhema. Getting into this area is what absolutely destroys the silliness of Open Theism. Everything within creation was in God's noumenon from beginning to end. And it's His volition as to everything unfolding, since His phaino is what gives all creation its phaino.