> “False slander” is not a thing.

It's only not a thing because, in the U.S., it's redundant. In other jurisdictions, it might be a thing, because there are places where a claim can be both defamatory and true.

This is more a question of English than law. Slanderous statements are false by definition.

There are countries where "false defamation" could be a term of art. Japan, for instance, where spreading rumors about someone that hurt their reputation is actionable even if those rumors are true. If your boss is having an affair, for instance, and you tell all your coworkers, he can successfully sue you.