It's a court system - the accuser must still show proof.
I don't think there is any dispute of the facts here though. Zille said a horrible thing that massively hurt the dignity of most South Africans. On a public forum. We all saw it. There's no possibility of a witchunt where there is abundant evidence and the accuser does not deny committing the act.
She may believe her assessment was factual (it wasn't - nowhere close to it, hell one of her examples "our independent judiciary", was actually created by the ANC in 1994 - we never HAD one under colonialism) but that has nothing to do with whether she harmed people's dignity or not.