Biology, race, and geography all influence humanity and therefore our cultural expression. Isn't geographical location a consequence of the need to adapt and survive?
For instance. Group A homo sapiens were adapted to life in Northern Europe during 6000BC, establishing a deep cultural, political, and communal based society that flourished, with all there symbolic representations of this time reflecting their success. However, 2000 years into this cultures history, a large-scale climate change causes devastating agricultural losses, and so the people simply move on (migrate) to 'greener pastures'. Do they take their symbolic representations of their society with them and pass this on through to the next generations of this particular culture in another part of the world? Wouldn't their cultural expression be forever changed not only as a consequence of geography, but also environmental factors, such as foods, climate, access to drinking water, and interactions with other peoples? If so, then biological, and racial influences are helping to shape Group A homo sapiens cultural expressions in their new location, not only a single factor. Anthropological studies do include geographical representations of human culture, but cover many other factors as well that influence or shape humanities ethnical, cultural, and spiritual differences.
Group A homo sapiens is my invention too, just a way to try and ask the question, not based on real history.
I assume that you are positing this possibility as a way of arguing that Diamond's thesis is invalid. If so, I think that you are missing two things. First, Diamond does not argue that all forms of cultural expression are dictated by geography. Second, Diamond's major thesis has to do with the differences between people on different continents, not between, for example, French people and Scandinavians.
With regard to the first point, Diamond is talking about economics and politics, not about culture. He is not saying that cultural and spiritual differences are determined by geography. He is saying that food production and, thereby, technological levels and population densities are determined by geography. These things allow some societies to dominate and others to be dominated.
With regard to the second point, the kinds of migrations you are talking about have tended to be within continents. Diamond's thesis is that geography caused Eurasia to be dominant, not that it caused England to become stronger than Portugal. Since Diamond is arguing about things on such a large geographic scale, the kinds of migrations you are discussing seem much less relevant to his argument.