Look at the median age in SA: 27.6 years. Barely over half of the median age in Italy.
You would not expect a huge wave of deaths in such a demographically young country.
Partial herd immunity aided by behavior changes seems a reasonable explanation. Remember that, if you adopt good anti-viral practices, herd immunity comes at a lower level. We may just be talking about a country of mostly young people, wearing masks, and spending time outside. Add in a bit of being afraid to gather for the last few months, and those 4 factors may explain the shape of the SA curve.
Good point on the prevalence of the SA variant within SA. If the SA variant is somehow still minor within SA, my theory is bunk.