“...does not entitle him to tell the jury whether or not a witness is telling the truth.”
He did NOT tell the jury whether Z was telling the truth.
He gave his OPINION on the veracity of Zimmermans statements. There’s a big difference.
That's the hallmark of an expert witness. Experts, and ONLY experts, are allowed to express opinion.
You have the last word. You don't have to believe me, and our argument won't benefit you, me, or the audience.
Indeed, you are correct.