The answer to this question can be found in the Prologue of Guns, Germs, and Steel. Specifically, it can be found on page 20 of the paperback edition of the book. On that page, Diamond says that the average New Guinean (he worked among them for over 30 years), struck him as being
…more intelligent, more alert, more expressive, and more interested in things and people around them than the average European or American is.
Diamond is saying this for two reasons.
First, he says that natural selection has worked to make New Guineans more intelligent but has not done the same for Europeans. He says that Europeans who die young have, historically, tended to die from things like infectious diseases. That means that Europeans who died young did so randomly. It was not as if being smart helped you to escape an infectious disease. By contrast, New Guineans who died young died from things like war or accidents. He says intelligence can allow people to avoid that. Thus, natural selection would have helped make New Guineans more intelligent.
Second, Diamond says that Europeans do not think that much from day to day. He says that they do passive things like sitting around and watching TV. New Guineans, by contrast, are always doing things and interacting with people. This helps their brains develop more than those of Europeans.
For these reasons, Diamond feels that New Guineans are definitely not any less intelligent than Europeans and are probably more intelligent.