In order to understand what Diamond is saying about the future of human history as a science, we need look no farther than the last sentence of the book. There, Diamond tells us that he believes
that historical studies of human societies can be pursued as scientifically as studies of dinosaurs…
In other words, Diamond is very optimistic that human history can be studied relatively scientifically.
Diamond understands that human history cannot be studied in a laboratory in the same way that something like chemistry or some kinds of physics can be. However, he points out that there are a number of kinds of science that cannot be studied in that way either. He lists (on p. 424 in the paperback edition) sciences such as astronomy, climatology, ecology, evolutionary biology, geology, and paleontology as sciences that cannot be studied in controlled experiments. Therefore, he thinks that human history can be studied scientifically just as those sciences can be.
Diamond suggests that historians should use “natural experiments” to try to study history scientifically. He feels that there are many natural experiments that could be used to help us understand history better and to study it scientifically. For these reasons, Diamond is optimistic about the idea of human history as a science.