You can find the answer to this by reading the Epilogue. There, Diamond is proposing that historians should try to treat their discipline more like a science. Diamond argues that the small scale things that you mention in this question have their place in short-term studies of history. However, he says that they are not relevant for studies of long-term trends like those that he studies in this book. Diamond says that historians can and should discuss issues such as how important individual people are in the short term. However, they should realize that individual people and specific events is not really relevant when one is trying to understand broad trends in human history.