You can answer this by reading in the Prologue. There, Diamond tells us there are two reasons for this hypothesis.
First, he says that New Guineans have actually needed intelligence to survive. Westerners, he says, have not really needed this for a long time. Therefore, New Guineans have been selected for intelligence and might be smarter than Westerners by this time.
Second, we can look at daily life. Westerners engage in a lot of passive entertainment like watching TV. New Guineans are always doing things as they grow up. This means that they are more mentally engaged and are therefore likely to be smarter.