In your description above, I presume such a dualism should be considered false, i.e. the opposition is "read back ontologically" and it shouldn't be. Am I reading you correctly?
And if I may follow through, I take it that there are dualisms that are ontologically fixed--not read back--into creation.
I think some clarification will be needed as to kinds of dualisms. The relation of hot to cold is a kind of dualism that functions differently than the relation of creature to creator or between temporal and eternal or between immanent and transcendent etc.
No, I am not the author and I am still working through to try and understand all of the nuances the author presents. Creation was declared "very good." The ontological status of man and creation as good was thus the only ontological status. Thus, it is a misconception to consider the antithesis (evil,sin,Satan) as a part of the original creation. Dualism is then giving the antithesis a necessary reality. Dualism gives the antithesis a certain structure within the world, a place of it's own. Antithesis is directional. It points us back to our original ontological status. The claims is that nothing in this world can be purely good or purely evil. The antithesis has layed claim on all parts of this world but the Thesis also is "reconciling all things to Himself."