I believe this is a Civil Trial and not a criminal trial.
I believe that you are correct. I believe the issue at hand is whether or not Trump defamed Carroll by calling her a liar.
If Trump did in fact rape Carroll, then Carroll’s accusation is true. That would make Trump’s act of calling Carroll a lair into an act of defamation.
If Trump did NOT in fact rape Carroll, then Carroll’s accusation is False. That would make Trump’s act of calling Carroll a liar into a simple statement of truth and not defamation at all. Also, it would make Carroll’s act of claiming that Trump raped her into an act of defamation.
While this is formally a defamation case, the question to be answered is: “Did Trump rape Carroll?” Who defamed whom all hinges upon that.