Plagiarism may or may not violate copyright law of particular countries, however only the copyright holder has a right to bring a lawsuit. There is no third party right of enforcement. It is not criminal and damages have to be proved.

Defamation, by falsely accusing someone of plagiarism, is also a violation of the law in many countries. In some places there is automatic damages (without having to prove it) and it can be a criminal offence. This is the case in Israel for example.

Thus Defamation is a far more serious issue than plagiarism.

If defamation is proved then downvotes received after the defamatory false plagiarism notification (eg by Hivewatchers) could well be considered by the Court as part of the damages.

Essentially Hivewatchers is a vigilante organisation that purports to uphold one law (copyright) but appears to regularly violate another (defamation).

Courts really don't like people taking the law into their own hands.

As I've previously advised, Hivewatchers is playing with fire and will sooner or later get burned.