You could also look at it this way: the FDLE worked to uncover more but were not able to find more due to the time that had passed. How would the investigation have proceeded if SPD knew TM was talking on the phone at the time of the altercation, or what might they have learned had they secured his vehicle as part of the initial investigation? The prosecutors had a date to speak with George but he changed his mind; what was he likely to be asked at that time? IMO the Sanford PD established in their final questioning session, the one where they finally play the non emergency call to dispatch that GZ mischaracterized a major part of his story, namely the way he traveled from near the clubhouse to the T intersection and that GZ was presented with his contradictions repeatedly and offered the chance to amend his statement but declined, thus forming the framework for the prosecution's strategy of attacking his credibility and demonstrating that he is hiding actions they can use to show a jury he had a different motive than the one he claimed for exiting his vehicle and for moving his vehicle. There is "the crime" and there is "the cover up." SPD arms to have found at least one of those, IMO. GZ told dispatch that TM was "at the clubhouse" when he came to approach GZs vehicle - he says the two things seemingly in the same breath. Yet later he claims this action by TM is one of doubling back from near the T and circling GZs vehicle, an action that is impossible to reconcile credibly with the recording. GZ also made the claim he was directed by dispatch to move from the clubhouse parking lot (in so many words) and this stands in opposition to the idea that he was already parked at the cut thru as the recorded call began. It seems to me that SPD concluded their interviews with GZ convinced he was pushing a false narrative at least in those regards, but lacking Dee Dee or a good eyewitness to the start of the physical aspect of the altercation knew the case for murder was weak but that GZ wasn't fully credible.
Somehow the special prosecutor is behaving in a manner that suggests they can take the case to trial and prove Murder in the second degree. What do they have that Serino did not? We don't know, but we can make a few guesses. perhaps all the prosecution needed was time to sift the clues.