But, to my ear, that sounds absurd.
The very first document produced in an investigation like this is the police report, completed by responding officers. In the officers' police report, they documented what the most likely charge would be if any subsequent investigation determined there was probable cause to arrest Zimmerman.
Here is the pertinent part of the police report:
They basically have to choose a classification for the death, for example, "Accidental", "Natural", or a variety of criminal classifications. Since there was another party involved and the responding officers do not do a complete investigation, they classified it as Manslaughter.
Every subsequent document produced would also carry "Manslaughter" unless it was changed at various stages. This would only be done if something happened such as Serino learning that Zimmerman and Martin had argued the day before, and Zimmerman was heard to say, "I'm gonna kill you one of these days". Then the report could have been changed to First Degree Murder / Homicide, for example. As it was, though, Serino left it alone.
Even if Serino or any other investigator later opines in a report that they don't believe Zimmerman's story, and presents that report along with the already settled title of "Manslaughter", it does not mean that the opinion expressed in the report and the title of Manslaughter are linked in any way.
All that matters is whether or not the investigator found that the facts of the case comported with the elements needed to charge the titled offense.
In this case, the facts of the case did not comport with the titled offense, therefore no charges were applied to Zimmerman, despite any opinions of truthfulness expressed by an investigator.
Any opinion by Serino that Zimmerman was being untruthful was essentially dicta, and not controlling for the purposes of charging Zimmerman.