Diamond should be characterized as an environmental historian because of the fact that his major thesis has to do with the environment. Diamond centers his whole book around the idea that the environment in which societies arise plays a huge role in determining whether those societies become powerful or not.
Diamond's major point in this book is that geography has a huge impact on history. He proposes that the number of domesticable plant and animal species, along with the direction of a continent's longest axis, is the ultimate cause of whether a society becomes powerful. These factors are purely environmental and Diamond is using them to explain some of the most important outcomes of human history.
Because Diamond puts such a huge emphasis on environmental factors, he can be characterized as an environmental historian.