The author in his claim states that the mammals of Kaliko Islands become extinct because of climate or other environmental factors and humans had no role in their extinction. Though the argument seems plausible it is based on many false assumptions.
The author assumes that since no site have yet been discovered with large mammal bones humans can have not hunted them. Just because no such sites are yet found, it does not mean that humans did not hunt them. This observation could have other possible explanation. First there is a possibility that such a site could be found in the future. Second, since bones of large mammals are very strong, it could have been used by the humans in making ornaments, weapons and even their houses rather than discarding them after killing the mammals. There is also less clarity about the statement that bones of large mammals were not found, it could mean that bones of smaller mammals were found, which would suggest humans have hunted them. If the author had provided details about the materials that were used by the inhabitants of Kaliko islands and improved the clarity of statement as to whether small mammal bones were found or not the argument would have been stronger.
Let us assume that the humans did not hunt the mammals. Then also humans could be responsible for their extinction. The author says that there is no evidence showing humans had any significant contact with the mammals. If there is no contact with humans as author points out, then also humans could have been responsible for their extinction. The humans on reaching the island might have cut down the forest for agriculture. This could lead to the loss of habitat for the mammals. The humans might have established their settlements near rivers, which could have obstructed the mammals use of water. So, in some way humans were making the life difficult for the mammals by destroying their habitat, restricting them access to water. All these could have led to their extinction. So, humans might be responsible.
If climate change indeed was the cause for extinction, why was the human population not affected by these changes. If there was a climate change human population would also have been affected.
Since the argument of author is based on many mistaken assumptions, it is not necessarily true that humans were not responsible for the extinction of mammals in the Kaliko Islands.