Diamond does not claim that Europeans are innately more intelligent than New Guineans. He actually posits that the opposite might be true. Diamond says that the leading causes of death historically in Europe, especially diseases, have killed people indiscriminately, without respect to intelligence. In New Zealand, however, Diamond says that people have died from "murder, chronic tribal warfare, accidents, and problems procuring food." More intelligent people would be more likely to avoid these causes of death. So New Guineans may be, Diamond suggests, genetically predisposed to be more intelligent than Europeans. The inequalities in wealth, technology, and global reach that exist between Europeans and New Guineans are not the result of inequalities in intelligence.
Source: Jared Diamond, Guns, Germs, and Steel: The Fates of Human Societies (New York: W.W. Norton and Company, 1999), 20-21.