This is a version of environmental determinism. It basically comes out of the Victorian "race science" with things like phrenology and it's bunk. It starts from a bad assumption and then works backwards. So, it starts by assuming Anglo Saxon protestant and Nordic peoples are the superior race and then tries to make up evidence to support it.

There was just as much democratic tradition in the Americas, Polynesia, Australia, and Africa, but b/c a decision was already made about those cultures the evidence was either ignored, or more often not even considered. Ideas of tribal communities with all power chiefs was assumed to be their system of governance and that was that.

It was mostly used as a justification of colonialism and imperialism. In the Americas it's also very closely tied to the justifications of genocide and land appropriation of the indigenous residents.

On top of that, it just doesn't make sense. Florida is also a peninsula. Korea is also a peninsula, south east Asia is basically peninsula after peninsula. There's mountains pretty much everywhere and the biggest mountains are in Asia. There's natural harbors along pretty much every coast and some places, like the Gulf of Mexico is basically one big harbor. The Americas were probably the most forested land in the world, and still are. No one is looking at Brazil for examples of democracy.

No respected historians believe it b/c it's so easy to find counterexamples.