I took a look just now at one of the hivewatcher comments. There is no direct statement that the post being commented on is plagiarism or that the poster is a plagiarist. It only contains a link to an external site that says "Source of plagiarism". Then it has a generic warning about plagiarism that also makes no direct comment about the poster or the post. I think it would be quite difficult to mount a legal case based on it. But it is possible the wording can be further improved.

Although for completeness, in some jurisdictions (eg Israel & NSW before recent reforms) a statement can be defamatory even if it correct. In these jurisdictions one must prove truth AND public interest to sustain the defence.

I don't think public interest would be difficult to show in the case of plagiarism, where you have a blockchain that rewards people for their posts. There is a clear public interest to the Hive community in this case in exposing potential fraud.