The question of when people first came to the Americans is very controversial among anthropologists, archaeologists, and others who study the topic. There is no consensus (and there was even less when Diamond wrote this book) as to when the Americas were first peopled. Diamond explicitly states that he is not certain on this issue. He says (pages 44 and 45 in my paperback edition of the book) that it is
uncertain when, between about 14,000 and 35,000 years ago, the Americas were first colonized.
Later, he does admit to feeling that the later date is more likely. He says (page 49) that his "feeling" is that pre-Clovis sites would be more plentiful if people had really been in the Americas for 20,000 years before the Clovis culture arose. But he again admits that archaeologists "remain divided on these questions (p. 49)."
Diamond, then, does not take a firm stand about when the first people came to the Americas. However, he tends to side with the "Clovis first" school of thought. It is worth knowing that fewer archaeologists hold to this theory today than when Diamond wrote this book.