I'd be interested in seeing what the arguments in the actual book are. I recall reading something about this before, that pointed to evidence that more diverse groups are less happy and have more conflict. And that people in general like being part of a group of similar people and having a group identity, humans being tribal creatures.
Anyway I'm not sure he's claiming western culture is homogenous. But it does seem like there is a cluster in culture space that is "western" and we don't have a better label for it.