As always, it depends on the point of view. When you look at a country from the outside (or as a whole), you'll mostly compare it to other countries and thus w

As always, it depends on the point of view. When you look at a country from the outside (or as a whole), you'll mostly compare it to other countries and thus will focus on what makes that country similar to itself (and different to other countries). When you look at a country by itself, or internally, then you'll start making divisions to see what different groups there are. Homogeneity is mostly a relative measure - the article seems to be trying to make (Japanese) internal homogeneity a universally (between countries) comparable measure.