Why can South Africa be described as a single, though not unified, country?
South Africa is one country, but it is not united. The country has always been divided by race and this has not ended with the end of the apartheid regime in the 1990s.
We can see this problem in current events from South Africa. The African National Congress has been in power since 1994, but not everyone is pleased with the way it has governed. It is now challenged to some extent by the Democratic Alliance, led by a white woman, Helen Zille, who was an anti-apartheid activist. This challenge has highlighted and exacerbated racial tensions. The DA's membership is mainly among whites and coloreds. ANC officials are portraying any opposition to the ANC as an act of racism.
This sort of thing shows how far South Africa is from being a united country.