My recollection is that Diamond's argument is slightly different. He argues that the east-west orientation of Eurasia meant that societies were more easily able to expand, because you experience relatively little climate variation moving along parallels than you do moving up and down meridians, which you are forced to do when you expand in a north-south oriented continent like America or Africa.

Similarly Eurasia had a wider variety of indigenous grains suitable for cultivation, and more large mammals suitable for domestication, and these could be taken with a population when the migrated (because of the aforementioned east-west orientation).

These advantages meant that populations were able to increase more rapidly, which necessitated an earlier development of agriculture, which lead to further productivity gains as improving food yields meant that people could spend more time developing more advanced technology.

The increased population densities had two further advantages - firstly, it meant that wars were more frequent, so Eurasians developed more advanced weaponry (the "guns" and "steel" of the title). Secondly, evolutionary pressures selected for resistance to diseases that could spread easily in crowded environments (the "germs").

So it's a little bit more subtle than just "our climate is inherently better for deep philosophical thoughts" or "our climate means that we are more productive workers".