There is a bright line distinction in trial, between an expert and a fact witness. In this trial, all of the investigators and police are fact witnesses.
It would be hard for me to do justice to a decent explanation, but the fact that a cop has 20, 30 or 40 years of experience does not entitle him to tell the jury whether or not a witness is telling the truth. That's for the jury to decide.
Not to say that Serino isn't a good judge of truthfulness, he probably is very good at it. But "truthfulness" can also be determined by a jury, without resort to an expert - that's the whole point of trial/jury, to have the jury weigh the evidence.
“...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.