Dominion sues Guiliani for defamation.

Any intentional false communication, either written or spoken, that harms a person's reputation; decreases the respect, regard, or confidence in which a person is held; or induces disparaging, hostile, or disagreeable opinions or feelings against a person.

They will have to prove that somehow and Guiliani will be able to defend himself. How would he do that?

By presenting and hopefully proving the validity of his claims, i.e. proving that his communication wasn't false, i.e. that voter fraud indeed has occured.

Just a hunch, but makes a lot more sense than presenting the evidence at the Trump impeachment.