It's more a hypothesis than a theory. It's also politically incorrect, and the soft sciences are pretty compromised these days.

Does Diamond say that in that book?

As I pointed out to some other commenters here, this article is the tail end of a series I did on this hypothesis. I just returned to it recently when I came across that map of ongoing war zones.

https://aninjusticemag.com/hotheads-and-cooler-heads-how-climate-decides-whos-free-cff649918552?sk=27117742305d88b0712dffd1e313a9ab

https://medium.com/grim-tidings/how-the-ice-age-turbocharged-human-intelligence-17353ea46ef9?sk=9cea478e9628d9a7eb115aec432b2bb0

https://aninjusticemag.com/does-global-warming-guarantee-autocracy-in-the-usa-151987a4256a?sk=333ca2257bc05671809e77915127beb4

https://medium.com/grim-tidings/does-the-nordic-model-work-only-in-a-cold-climate-595e9fdfe917?sk=9e75a7cbc3979f5eb31be087a96fc1c2

https://medium.com/grim-tidings/how-africas-climate-bakes-its-ethnic-divisions-b16f68a1d81d?sk=d9b79f12a474ea18837995d9cece16bf