South Africa was somewhat more paranoid about the Red Menace than other democratic/capitalist nations (anecdotal source: vacationing family members spent a night or two in jail for re-entering the country with The Communist Manifesto in their luggage... a text they were studying for school).

But there were some reasons for this.

In part, they were nervous because they were actually fighting a shooting war against communists in Angola for more than 20 years. The Soviets and Cubans were involved, and I believe there were Chinese "advisors" as well.

At about the same time that started heating up, the CIA (and MI6) was doing away with Patrice Lumumba (and, as far as I'm concerned, assassinating Dag Hammarskjold)... illustrating at least that sub-Saharan Africa was, just as much as Southeast Asia, a "hot zone" in the Cold War.

Within South Africa, the ANC had the support (nominally at least) of the Soviet Union since the 1920s, so apartheid authorities had more than just "those countries out there" to worry about... not that there were necessarily plans to nuke the townships or something (were there?), but they had plenty of reasons to be nervous about Communists.

South Africa and neighboring countries have a lot of mineral wealth ( gold, diamonds, uranium), and the Cape still defines some important shipping routes - meaning, there's a strategic value to South Africa, too.

Signing up for the Mutually Assured Destruction deterrence made a fair amount of sense, from that perspective.

On dismantling them — apartheid was about to end, and the apartheid government that was leaving did not want the new government to be a nuclear power (even a small one). They only had six weapons or so, so dismantling them was not a huge effort.

Edit: I am on my phone and do not know how to edit posts via this app, so I am putting this edit as a comment.

After the word club add: "have nuclear weapons"