Vela Incident (opens in new tab)

That sounds a bit like Israel’s policy — Although officially Israel’s policy is one of deliberate ambiguity, if they have nukes then they are doing exactly what you just said; and if they don’t, then presumably the scientist that they imprisoned for revealing the existence of the nuclear program was a stooge designed to make them look scary without any of the cost and complexity of actually bothering to make any nukes of their own.

Not so weierd... Develop in secret. Once you have it your enemies knows in any case. And if you can maintain plausible deniability it is much easier to keep developing / much less public scrutiny, especially if you already are operating a highly unpopular apartheid state.

I remember that during the war with Cuba in Angola the South Africans deployed their nuke on the border, the Cubans knew this and as such the South African border was never threatend.

It's just the Art of War. If you don't have a strong program yet, say nothing about it and let your enemies wonder about your capabilities. Just because they may have one undeclared test doesn't necessarily mean they could succeed with a public MAD strategy; they may need many more nukes than they currently have.

If they say they have none, their enemies might think there's a chance they actually have a lot, or at least enough to cause serious damage. And there's the deterrence. If they say they have a lot but don't demonstrate it, their enemies will probably think they're bluffing and will demand proof. I think it only makes sense to go public when it's a gun that can be pointed directly at someone's head, rather than a few of their toes. It's probably the best way to project a strong posture despite not actually having one.

And more practically, it lets them develop the program with less chance of other countries (including allies) complaining about it or condemning it, which could potentially slow it down or stop it before it's sufficiently developed.

They were intended to defend against conventional attacks by their neighbors, not deter other nuclear powers. It would still be nice to deter such attacks, but it also works to reveal them after the attack begins, or just use them. I imagine they didn’t think it was worth provoking condemnation from the international community by revealing them.

Revealing that you have a weapon as strong as the nuke is the stupidest thing to do for a military force

Did we read the same article in wikipedia? It quotes many many sources saying it was not a nuclear test.

The quotes that it was a test were from journalists, and from people who "heard" about it. None from actual scientists who studied it.

Also that the 'glitch' happened to occur in very close proximity to two uninhabited* subantarctic islands controlled by South Africa - and that sheep in Western Australia showed higher levels of iodine 131.

*Permanently uninhabited, but there's a research base there.

Red mercury is not a thing. Or rather, it's a very pervasive repeated hoax that plenty of people have fallen for.