The essence of this law is that that you have no "duty to retreat" from an assault if you are in your house or vehicle; which isn't the case here.
(Normally, if you are out in public and are attacked, you are supposed to retreat if possible before using deadly force in self defense. Not so now if you are in your house or car).)
Zimmerman never had a chance to try to retreat, he was flat on his back. He was under unprovoked attack, and he used lawful deadly force to stop the attack. "Duty to retreat" had nothing to do with it, under the circumstances.
The law was changed a few years ago. Now, if you have a right to be in any public place you can claim SYG. No longer do you have to be in your home or vehicle.
I understood it the way you described as well.
I thought the person that wrote the Florida Stand Your Ground legislation has said that it doesn’t apply to Zimmerman.
Not exactly. It goes further. While I don't know the specifics of the law state to state as many states now have a form of it, the significance of "Stand your Ground" laws is that it protects a citizen in any circumstance against prosecution where lethal force is used in self defense. They specifically abolish a requirement to retreat in any circumstance when threatened, hence the name "Stand Your Ground."
The courts have always recognized a citizens right to defend themselves with lethal force in the home or in their car when faced with an imminent threat. Laws often trapped citizens because they had to prove what they thought about an attackers intentions (just give them your wallet and you wouldn't have been harmed for example). Then citizens were prosecuted on the decisions they made based on what they should have thought immediately following initiation of the event. Prosecutors argued that they acted before they knew whether the perp had a weapon for example. "Stand Your Ground" makes self defense more objective and less subjective.
This case is relatively unique in that GZ's actions as an attentive citizen led to the confrontation that resulted in the necessary use of deadly force. Had GZ ignored the suspicious individual and went on his way, there would have been no confrontation. But GZ did nothing illegal and should therefore not be responsible for TM's death.
Not so under the protection of "Stand Your Ground" laws. You can stand your ground in any circumstance in which you are legitamatly faced with a threat of harm or death.