The writer for the liberal publication only hints at the answer to how ANC's leadership "sold out", or more accurately, was co-opted by mining moguls in South Africa.
...by late 1993 big business strategies – hatched in 1991 at the mining mogul Harry Oppenheimer's Johannesburg residence – were crystallising in secret late-night discussions at the Development Bank of South Africa. Present were South Africa's mineral and energy leaders, the bosses of US and British companies with a presence in South Africa – and young ANC economists schooled in western economics. They [the latter] were reporting to Mandela, and were either outwitted or frightened into submission by hints of the dire consequences for South Africa should an ANC government prevail with what were considered ruinous economic policies. (my emphasis)I don't think that they were "either outwitted or frightened into submission". I think that they likely saw that they personally could reap big benefits in terms of wealth and power by cooperating with the capitalists' agenda.