Is there any reason to believe that having a large number of previously infected individuals is unique to South Africa?

https://covidestim.org/ provides estimates of the percentage of previously infected individuals by US state. I happen to be in Massachusetts, for which they estimate around 50% previously infected, with error bars. So a bit lower than South Africa, but not too far in the big scheme of things.

The UK provides some estimates of seroprevalence in blood donors (thus a skewed sample): https://www.gov.uk/government/publications/covid-19-vaccine-... (Figure 3). These estimates aren't quite what you are looking for, but one test is sensitive to previous infection and estimates 20% previously infected, whereas another test is sensitive to (infection OR vaccine) and estimates ~100%. Basically, in that population, pretty much everybody has some kind of antibodies.

To answer your question, I would say that SA doesn't look particularly unique.

SA isn’t unique, but it’s the location of the study this post is about, so the stats there are relevant.