SYG *would have* not had anything to do with the Zimmerman trial, but the really bad “judge” inserted it into the Jury Instructions, so in fact it did.
The judge was probably obligated to inform the jury of all details of the law which would be necessary to decide if justified self-defense was at issue.
For example, if the jury had determined that Skittles was attacked, then the helpless little boy would have had no obligation to retreat and would have been justified in battling Zimmerman. Whether or not Zimmerman was sufficiently helpless to recover his right to self-defense would be a question for the jury.
You don't get to shoot an attacker if they are no longer a threat. You also don't get to pound an attacker's head into a concrete sidewalk if they are no longer a threat.