You can refute a statement or proposition (which might be an allegation of wrongdoing). You can mount a defense without refuting any allegation; for example, it's usually a defense to any allegation to say that "It wasn't me", without claiming that the event didn't happen.

In the UK, it's usually a defense to say that the act was justified, even if it happened and you did it. I think it's usually a defense to claim that you had no choice.

I suppose my persnickety comment was to the effect that if the claim is true, then there was no slander; you aren't innocent or guilty, because the alleged act simply didn't happen.