An alternative question I might have put to him was whether he could make the argument for apartheid about as well as a competent defender of that system could.

That's a wrong question to ask. He is not an expert. The right one would be

whether a competent opponent could make the argument for apartheid about as well as a competent defender of that system could.

Shouldn't his arguments screen off his authority? Isn't that the whole point of arguments?