I have never heard of this. The African National Congress (ANC), which fought against Apartheid for decades, had close ties with communist regimes in Angola and Mozambique. Those countries in turn had Cuban help in the fight against (South Africa-backed) right-wing rebels.

The South African government at the time was rabidly anti-communist -- think Eugene McCarthy and the red scare in the U.S. -- so I doubt they would have much to do with Cuba.