Its not really a stand your ground case if the facts are accurate. Stand your ground only says you don’t have a duty to retreat first.
Zimmerman was attacked and the guy was sitting on his chest trying to kill him. Now if Zimmerman had shot him on the approach stand your ground might have been applicable.
That's actually a common misconception. Florida's stand your ground law contains (in essence) two parts. First is the part your'e referring to, clarifying that there is no duty to retreat before defending oneself.
The other part grants immunity from prosecution to people who act in self defense, and allows for pre-trial hearings to determine whether a defendant is immune from prosecution. While these hearings are commonly referred to as "stand your ground" hearings, they can happen in any self-defense case, regardless of whether the classic duty-to-retreat "stand your ground" issue is present. Zimmerman waived his right to such a hearing (or, more precisely, deferred such a hearing until the trial itself).