This is a strong accusation, but it has the evidence behind it. The most recent of which is a report published at the UN Human Rights Council[1], but also the case filed at the World Court by South Africa in December with addenda added in March[2]. The evidence for this claim is both public, overwhelming, and has been filed at the world’s highest court.
All that said, I actually didn’t make the claim here—though I have elsewhere—I merely said we should be open to the possibility that this is the case.
1: https://www.ohchr.org/sites/default/files/documents/hrbodies...
2: https://en.wikipedia.org/wiki/South_Africa_v._Israel_(Genoci...