Why it was not South African special police? Usually you start from evidence that points to a suspect. I am not aware of any.
So I understand that many people admitted to the murder. In South Africa, there was a specific admission to the murder by Craig Williamson, who is a ex-special agent. I don't think that here there is the same phenomenon by which multiple people admit to the murder in a delusional way.
This admission was done as part of the TRC (Truth and Reconciliation Commision). There is no reason here not to believe them. The only thing I can think of is delusions of grandeur, but like I say, I don't know of many TRC revelations being untrue.
Is there a simple argument why it was not South African special police? I understand that the Swedish police decided it implausible, but did they explain why?
I think the amount of paranoia and turbulance in SA during the 1980's and the other operations of the special army/police factions make it a somewhat believable possibility. The other thing to keep in mind is that during that time a lot of people were already acting in a sort of "rogue" way. There are always people that go to the radical extreme before a change in power.