If we look only at this chapter, Diamond is clearly an environmental historian.
The subtitle of this chapter is "Why did peoples of some regions fail to domesticate plants." If Diamond were a cultural historian, he would examine the cultures of the areas in order to explain why the people failed to domesticate plants. But Diamond does not do this. Instead, he looks at environmental reasons to explain why some areas' people did not domesticate plants. He points out, for example, that northern Australia had only two grass species with large seeds and the rest of Australia had none. This, he says, explains why people in those areas did not domesticate plants.
Diamond is an environmental historian in this chapter because he uses environmental explanations for the phenomenon that he is trying to explain.