Last bit first: when you write your book, you can title it "Trade, Germs, Guns, & Steel" and do Diamond one better. ;)

I'll give you MY answer to the rhetorical question.
The focus of geography is on the physical; from there we widen our view.
The focus of anthropology/history is on people(s).

Yes, they all overlap. It is the central focus and scope that differs between them.

(An aside, I think that historians would bristle at your suggestion that they study "records", as if records are studied for their own sake or that they constitute the scope of a historians tools.)

Does Diamond explain why the technical advancement in the Western Hemisphere occurred in tropical MesoAmerica rather than Temperate North and South America?

Europe seemed to have both a climate that gave surplus resources and migration of conflicting people groups that gave competition for those resources as a motivation for technical advancement ... and most of the real advancement in technology we owe to labor shortages caused by plagues spread from contact with other lands that got the ball rollling.

Sustainable TLs (atpollard's House Rule)
 

My RULE OF THUMB for sustainable TL is that Sustatainable Tech Level is equal to the basic unit of organization for the Population.

• A Family (POP 0 = 1-9 people) can sustain TL 0 with no outside resources, like a hunter-gatherer group.
• A Village (POP 2 = 100-900 people) can sustain TL 2 with no outside resources, like a self-sufficient Medieval Manor.
• A City (POP 5 = 100,000+ people) can sustain TL 5 with no outside resources, like an Industrial City supporting factories and the farming communities.
• A Nation (POP 8 = 100 million+ people) can sustain TL 8 with no outside resources.
• A Starfaring Community (POP 11 = 100 billion+ people) is needed to sustain TL 11 with no outside resources since POP 11 is typically beyond the capability of a single planet to support. It can barely be done on a single world with high urban densities, massive space resource extraction, and vast orbital agriculture all drastically raising the cost of living, but most TL 10+ worlds require interstellar trade to survive.

Higher populations at lower TLs simply indicate that there are multiples of the basic unit that are more isolate than working together. For example, the Grain Empires (Egypt, Greece and Rome) were TL 1 or TL 2 because the dominant unit was Hundreds of people (Small farming community that grew the grain that supported the empire) and 80-90% of the population lived in these small communities. Note that where you had larger populations like the Thousands (POP 3-4) of the Greek City States or the Millions (POP 6) of Rome, you start to see innovations common to later TLs appearing like the Mechanical Devices and Libraries of Greece and the central heating, running water and indoor plumbing of Rome. They are still limited by the majority of the population living in POP 2/TL 2 Agricultural communities.

No man is an island.

Cultural and genetic appropriation is a natural occurrence, as would be technological.

The Chinese invented printing and gun powder, but internal socio economic political pressures ensured that they could never fully exploit them.

The Mediterranean served as a communications hub, and when those civilizations that bordered there stagnated, their intellectual properties were exploited by more robust cultures, packed full of ambitious opportunists.

Have you read Fernand Braudel's histories of the Mediterranean? These might be up your alley.