I’m not sure that’s quite the situation. If you told someone I was a fraud, then they told others I was a fraud, that person wouldn’t be protected.
However, RNZ is not making claims about they guy, they would be saying that Winston made claims about the guy, which is a true statement easily provable.
I’d guess that this might be a line RNZ just aren’t willing to test since court cases are expensive even if you don’t lose.