SYG does more than give one the right of self defense if you could run away but didn't. It also protects one from civil suits by the family of the perp/victim.
Just because he was having his head smashed on the concrete and couldn't run away, Zimmerman is still protected by the state self defense law, which includes the protections of SYG.
I was not aware of the “can’t sue the guy” aspect.
My point is that if it came down as Zimmerman claimed, he was within his rights even without SYG. If SYG protects him from lawsuits, so much the better.
The point is that under ordinary self-defense laws Zimmerman still could not have been charged without probable cause which they initially did not find. The lawsuit protection aspect is irrelevant to that.