Why is the exchange between Benedick and Beatrice a mark of true love?
I'm not sure which exchange you are referring to...
The exchanges between Benedick and Beatrice in the first half of Shakespeare's Much Ado About Nothing are witty banter (verbal irony: sarcasm, overstatement, and understatement) that show that their two senses of humor make them soul mates. Really, their verbal "battle of the sexes" attacks on each other are decoys to mask the true love that they feel. It is said that if a couple can laugh together, then they can stay together.
Later in the play, Beatrice summons Benedick to act on his words and challenge Claudio for defaming Hero, her cousin. Actions speak louder than words, and Benedick's challenge in defense of a man's slander of a woman is a testament to how far Benedick has overcome his sexism and how much he really loves Beatrice. In this exchange, Benedick effectively marries Beatrice: