Okay, so from what I can tell, Wright is just playing semantics with the word "purpose," and that's all the latter part of the argument amounts to - a lot of sound and noise over an intentionally bad definition.

He gets Eliezer to describe some natural thing as "purposeful" (in the sense of optimized to some end), then he uses that concession to say that it "has purpose" as an extra attribute with full ontological standing.

I guess he figures that if materialists and religionists can both agree that the eye has a "purpose," then he has heroically bridged the gap between religion and science.

Basically, it's an equivocation fallacy.