Put it more simply still. To be bad, he must exist and have intelligence and will. But existence, intelligence and will are in themselves good. Therefore he must be getting them from the Good Power: even to be bad he must borrow or steal from his opponent.... Here, we have a switch between two different definitions of "good". Existence, intelligence, and will are "good" in the sense of "desirable" or "beneficial"; the distinction between the two powers posited by dualism depends on "good" in the sense of "morally upright". The two concepts can exist independently of one another -- one need only cite an example of a real, intelligent, and/or strong-willed advocate of an evil cause (e.g. Josef Stalin), or a fictional, stupid, and/or easily distracted advocate of a good cause (e.g. Inspector Clouseau).
Here, we have a switch between two different definitions of "good". Existence, intelligence, and will are "good" in the sense of "desirable" or "beneficial"; the distinction between the two powers posited by dualism depends on "good" in the sense of "morally upright".
No, the switch wasn't
her. Perhaps you missed it. Lewis made the switch further up and explains the distinction. At this point of your criticism, Lewis was presenting what we may conclude.
Have you read Lewis' The Abolition of Man at all? I ask because Lewis demonstrates there too why he expects your question.