I often wonder about these types of theories. They are often hard to prove or disprove. It is fun to try and come up with a theory that is equally as plausible given the information provided.
To your theory on art, could art be defined as anything which a group of people would define as art? Or potentially anything valued by a group seemly without functional value? It seems to be a balance between simplicity and informational content.
Acceptance by the art community is a key factor, possibly the most important. Other factors include artistic intention, aesthetic qualities, and being considered based on what is already considered art.
These hard to prove theories, such as Jared Diamond's and in the reviewed book, exist in large part to counter the highly convincing and data driven eugenic theories. The eugenic and IQ based theories are politically unacceptable. "A Farewell to Alms" and "IQ and the Wealth of Nations" lay out very convincing and heavily researched cases that explain different national economies very well. The explanations are just not PC.
To your theory on art, could art be defined as anything which a group of people would define as art? Or potentially anything valued by a group seemly without functional value? It seems to be a balance between simplicity and informational content.