We are more likely to talk about what person an author writes in when we are looking at fiction, but it is still possible to answer this question. In general, Diamond writes in the first person. He also writes in the 3rd person a lot, but that is because he is typically writing about things that cannot really be written about in the 1st person.
There are many times when Diamond uses the third person. He writes about historical figures such as Pizarro or George Washington using their names. He writes about ideas in the third person. Thus, he says things like (p. 277 in the paperback edition) “The remaining way for kleptocrats to gain public support…” He does not say “I think that the remaining way” or “I believe that the remaining way…” Instead, he writes in the third person.
However, Diamond more typically is writing in the first person. He does so when he is talking about personal experiences. For example, on p. 277, he says “…I happened to be visiting New Guinea’s Ayau people…” Even when not narrating personal experiences, he tends to speak in the 1st person. For example, on p. 276 he says “We consider George Washington a statesman…” rather than “George Washington is considered to be a statesman.” This sort of first person writing is common in this book.
Thus, Diamond writes this book in the first and third persons.