This excellent book links together science and history through the way in which Diamond appeals to both disciplines in order to justify his thesis. What is unique about this book and the author is the multi-disciplinary nature of his argument. He appeals to so many different disciplines all of which are shown to be necessary in terms of supporting what he is trying to argue.
Historians do take a scientific approach to history. However, people cannot really be reduced to scientific explanation. We can make generalizations, but people will continue to be unpredictable. The same can definitely be said about studying history. No matter what was base our theories on, sometimes we will never know.
He is saying that historians have to become more like scientists. He says that they have to start using "natural experiments" to prove points about major trends in history. He says that, if they do so, their discipline can become more objective and more like a science.