I think naturalism as James defines his terms is demonstrably wrong, and I appreciate his effort, but something strikes me as not quite right about his argument. Unfortunately I'm not sophisticated enough to put my finger on exactly what feels wrong. It just makes my spider-sense tingle.
It doesn't really matter if he defined naturalism in a way you wouldn't. I also think of naturalism differently (more in line with M. Rea's book, World Without Design). If James Anderson's argument is sound, that alone we be significant even if we were to replace "naturalism" with "physicalism" or any other ism.