SYG has nothing whatsoever to do with the Trayvon/Zimmerman incident. It was not used by the defense, and simply does not apply. The T/Z situation was a straight legitimate self-defense (in fear of death and unable to retreat). Such a defense is applicable in ANY (even the most rabid anti-gun) state. Of course, in the anti-gun states, the weapon of defense probably won't be a firearm....but the legal doctrine applies.
You are correct but the fact Trayvon backtracked, stalked Zimmerman down and then attacked him show that retreating wouildn't necessarily accomplish a thing and may put you in a more vulnerable position. Thus, his actions do show the necessity of SYG.