Diamond is trained as an physiologist, an evolutionary biologist, and a geographer. He has no formal training as an historian, but his approach in Guns, Germs, and Steel can certainly be categorized as that of an environmental historian. Many critics of the book, in fact, were environmental historians who claimed that Diamond underplayed human agency in his attempt to show the importance of geography in human development. His next popular book, Collapse, demonstrated that he had, indeed, thought a lot about the interaction between culture and the environment. So Diamond has certainly become what could be called an environmental historian, a field which is beginning to attract more and more trained historians as well as a few natural and social scientists.
In this book, Diamond is being a historian. Diamond is drawing on all of the disciplines you mention to find the evidence for his theories. This is what historians do. Historians need to find evidence from any discipline that is relevant. Therefore, Diamond is being a historian who draws from the findings of the other disciplines you mention.