To present those articles as 'the voices of South Africans and Arabs' instead of some South African and Arab voices are super misleading.
The African National Congress openly supports the BDS movement, perhaps they would be more indicative of the 'South African' voice than the stuff you found on stopbds.org
The parallels between Israel and the South African situation is not a new comparison, while Mandela never used the word apartheid to refer to Israel, he compared the situations extensively and spoke about how the apartheid regime worked closely with Israel. Some quotes from him:
'We identify with the PLO because
just like ourselves they are fighting for the right of self-determination'
'Our freedom is incomplete without the freedom of the Palestinians.'
Here are articles by two of Mandela's grandchildren using the term Apartheid to describe Israel:
Zwelivelile Mandela says Israeli apartheid is the worst form of apartheid ever witnessed.
My grandfather Nelson Mandela fought apartheid. I see parallels with Israel