De Klerk similarly had a great moderating influence. The alternative to the nonviolence of him and Mandela would have been a racial civil war in which tens of thousands died.
Rick Stengel on Mandela, via FAIR:
> One of most interesting things he ever said to me was this idea of nonviolence. Remember, we compare him to Gandhi, we compare him to Martin Luther King. He said: “I was not like them. For them, nonviolence was a principle. For me, it was a tactic. And when the tactic wasn’t working, I reversed it and started” –that’s a very important difference.
After his release in '90, he visited Castro and heaped praise on him for that among other things. There's no indication that he regretted that support for Cuba's direct military involvement in open warfare later, as far as I know.
Here's an excerpt (with links to the full speech) of Mandela's speech on the subject at a commemoration of the start of the Cuban revolution in 1991:
https://www.democracynow.org/2013/12/11/nelson_mandela_on_ho...
(I have plenty of issues with Castro, but it's worth understanding why he was seen as we seen by African leaders, and how we was seen by people like Mandela also elucidates those leaders own views)
Renouncing violence against a nuclear-bomb-equiped adversary is beyond foolish, strategically speaking. At the micro end, the same regime was not beyond shooting dead (unarmed) protesting high-schoolers less than a decade before the offer. Renouncing violence wasn't a viable option, but perhaps they hoped 2 decades imprisonment with labor had softened Mandela.
Given South Africa's geography, why do you think the apartheid government had (prior to the offer) developed its nuclear program? Hint: there was no parallel program for long-distance nuclear-payload delivery.