The answer to this can be found in the Prologue of Guns, Germs, and Steel. It is found on pages 20 through 22 of the paperback edition of the book. Diamond gives two reasons for saying that he thinks that people in New Guinea are smarter than Westerners. He does so to refute the idea that Western domination has come about because Europeans are smarter than New Guineans and other non-white people.
The first thing that Diamond says is that natural selection does not help make Europeans smarter but does do so for New Guineans. Life in Europe has been relatively easy for so long that most people died from epidemic diseases. There is no intelligence needed to survive an epidemic disease, just luck. Therefore, the people who lived to breed in Europe were not smarter than those who died. By contrast, intelligence helps a person stay alive in New Guinea long enough to pass on their genes. As Diamond says
Intelligent people are likelier than less intelligent ones to escape those causes of high mortality in traditional New Guinea societies.
In short, natural selection works to make New Guineans smarter but does not do so for Westerners, Diamond says.
The second argument Diamond makes is that New Guineans’ daily activities make them think more than Westerners do. Westerners do a lot of passive things like watching TV, Diamond says. By contrast, New Guineans are always doing things like interacting with other people that make them use their brains more.
These two factors, Diamond argues, make New Guineans smarter than Westerners.