This is how this reads to me: Guiffre said she was ordered to have sex with Dershowitz. It doesn't say that it happened. Dershowitz "defamed" her by calling her a liar. It sounds like he's taking her story that she was ordered to have sex with him as an accusation that it happened. So, she's suing him for defamation for calling her a liar (not for underage sex). And he is countersuing, calling her a liar for accusing him of having underage sex.
This is how this reads to me: Guiffre said she was ordered to have sex with Dershowitz. It doesn't say that it happened. Dershowitz "defamed" her by calling her a liar. It sounds like he's taking her story that she was ordered to have sex with him as an accusation that it happened. So, she's suing him for defamation for calling her a liar (not for underage sex). And he is countersuing, calling her a liar for accusing him of having underage sex.