If they had tried to get the other party to comment at the time of writing the original article I think that would be fair enough, but they didn't. Apparently after the story was published they were sent an email from

> someone who said he was representing the person NPR had identified as the "author" [of the purportedly slanderous website]

and then they retracted it. (https://www.npr.org/sections/ombudsman/2018/04/12/601650762/...)