Binary opposition is a linked pair of related opposites.
the existence of God does not imply the existence of Satan.
Proof? God pre-existed Satan and created him.
Further proof? Further proof, other beings exist besides God and Satan. One doesn’t have to be one or the other. One does have to follow one or the other, if both exist.
which brings up the primary binary question: God either exists, or he doesn’t. There is no in-between. There are lots of things between God and Satan.