Stress, imo, is a state of mind that is not directly attributable to the factors measured in this methodology.
Are bushmen more stressed than air traffic controllers for example? You can live a happy life without a cent to your name.
Carrying on with bushmen vs ATC (note I strongly suspect the average bushmen is happier than many of us), corruption doesn't affect one more than the other and income inequality, GDP per capita, unemployment favours the ATC, as does life expectancy probably given one has better access to modern medicine.
The only factors in that list that could be favouring bushmen is homicide rates (when you literally live in the middle of nowhere with nothing to your name odds are nobody wants to kill you) and urban air pollution, given that they don't live in urban areas and experience much less air pollution.
Yet I'll maintain bushmen are less stressed, so either I am entirely wrong or this methodology is.
Comparison fail! Technically bushman live in a different world. A somewhat more relevant comparison will be to look at South Africa vs the average nation which took part in the 2nd world war? Judging by the number killings per capita, SA comes out on top