When Diamond proposes that history become more scientific, he is talking about the history of broad processes in human history. He is not talking about the details of what we might call "micro" history. As he says, he is trying to look at why Europeans conquered the Native Americans, not why Kennedy won the 1960 election.
So, Diamond hopes to learn more about the larger scope of human history. He hopes to be able to discern "general principles" (as he says on p. 421 of the paperback edition) in history. He wants to be able to answer questions about major trends and factors that affect the whole of human history. He is trying to learn more about the "big picture," not about the smaller details of history.