Wouldn’t that be rewarding the prosecution for their own ineptitude?
I suppose you can look at it that way. But the Q&A came under a cross examination, so the state didn't elicit the answer itself. I do think they could see it coming as soon as Serino answered "could have been a pathological liar, or could have been telling the truth." Both of those are ultimate opinions, even though he hasn't told the court which of those two he leans toward. The state should have objected on the spot.
Funny thing is that the state filed and won a motion precluding exactly this type of testimony from law enforcement, and should have been on guard for it.