Also, I would like a simple argument to refute the assertion that Chalcedonian Christology is the foundation for the doctrine of theosis.
I don't think it needs a refutation, as much as it needs an explanation.
The author of the book you are reading is a Chalcedonian, and he is of course addressing Chalcedonian readers. Chalcedon and its language is how they frame their Christology, so it is natural that he should express himself in this way. It doesn't mean theosis was invented at Chalcedon, or was even first developed there.
Theosis is really based on Orthodox Christology, as has been expressed through the ages by all our theologians, including St. Iranaeus, St. Athanasius, St. Cyril, St. Dioscoros, and so on. In fact, it is my understanding that St. Dioscoros first used the four qualifiers, "without mixture or confusion and without separation or division," and that he also used the analogy of the iron in the fire in one of his letters. (Mina, or someone else, correct me if I am wrong.)
The Chalcedonians, when speaking of theosis, will go back to Chalcedon and its definition, since that is where their Christology was most fully expressed. As has been pointed out countless times by wimpy ecumenists like myself
, the Christology of the Chalcedonians (at least today) is pretty near, if not identical to our own. So I wouldn't worry about the author identifying theosis with Chalcedon. We all know it existed before Chalcedon, but he is just using Chalcedon as a frame of reference.