He's obviously talking about differences which are small by human standards. The rest of the paragraph:
> If you want to know who someone in former Yugoslavia hates, don’t look at the Indonesians or the Zulus or the Tibetans or anyone else distant and exotic. Find the Yugoslavian ethnicity that lives closely intermingled with them and is most conspicuously similar to them, and chances are you’ll find the one who they have eight hundred years of seething hatred toward.