Answering the question as written and without any particular comment on the example given, yes, it can be manipulative to respond to allegations with a counter that the allegations are slanderous, but it’s not always manipulative. Though it does highlight the gap between what someone says and what they mean.

Some folks will argue that the term “slander” has lost its original meaning but I still ascribe to the legal definition which says it’s a form of defamation. That is, a falsehood being perpetuated about someone, regarding something that can be proved one way or another. Without defining the term, no one could sensibly answer the question here.

So does this mean when person X asserts slander, they’re saying they have a potential lawsuit? Possibly, but that’s the rub: “slander” is now colloquial shorthand for “all those things they said about me are lies”. It’s natural to refute unsavory descriptions about oneself, even in spite of attached evidence, so sometimes calling something as slanderous is a knee-jerk reaction, akin to saying “ow!” after stubbing one’s toe on the bed post.

But that’s the most charitable view. “Slander” can also be a rejection of the validity of the evidence, and that’s more a symptom of the “post-truth” era we’re currently in, where controversies are more newsworthy than the truth they purport to stand upon. In that sense, labeling allegations as slanderous is manipulative because it serves the purpose of misleading the audience, or encouraging them to not bother vetting or even looking at the evidence at all. And sometimes you can’t blame the audience: fact checking is difficult and boring.

I bemoan the use of legal terms in colloquial speech, as terms of law must be exact and precise, while colloquialisms must be free to express broad thoughts and ideas. But since laypeople are rarely asked to consider if a comment is actually legally actionable, and most off-the-cuff commentary isn’t, I won’t dwell on that aspect.

Sometimes it’s the original allegations which are genuinely manipulative, and it’s not manipulative to point out manipulation. Other times, no possible reading of the facts can save the remaining reputation of a genuinely awful person.

But what I will say is that a simple response of “it’s slander!” alone is not persuasive, IMO. If someone wants to be believed, they’ll have to put more effort into their defense than that, insofar as public perception is concerned. But if the evidence is bad, this shouldn’t be very hard to do.

The public and media might also bear some responsibility, if they will so easily equate “both sides” as having valid points when there’s an avalanche of evidence on one side and only bad evidence on the other. But other times, a factual question can be reasonably interpreted differently, and that’s fair too.

So I’m sorry to say that there’s no clear cut answer, whether it’s always or never manipulative. I’m not a lawyer, but their common adage is “it depends”.