The fact that the cop did not observe any of the tells associated with deceit was not speculation. The only thing that would be remotely speculative would have been whether GZ could have been someone who could lie freely without it bothering him in the slightest; I think GZ's other behaviors make clear that he was not such a person.
It might have been possible to put the officer through a questioning about his police training which would then create the legal basis to NOT ignore the statement, but for whatever reason Mark O’Mara probably felt that would be a sideshow.
Yes, one would really need to multiply conspiracy theories to paint GZ as an easy liar. The few missteps he has been allegedly caught at (re the financing of his defense) are still matters for a hearing, and they involved vague questions.