It proves that he is lying. They played that Hannity interview, and it was a long one, right in court. He actually laughed and said he had never heard of stand your ground. He must be fabricating other things in his version of events.
It would be important if he was using stand your ground as a defense but he's not. Other than that it's just something he wasn't thinking about at the time. If legal procedure was something he dealt with every day he might have remembered better.
There is just no way the prosecution can say "Mr. Zimmerman you knew about stand your ground when you pulled the trigger. You intended to use this as a defense didn't you."
Looking back this is probably why the defense attorney waived the immunity hearing. It was a masterful use of strategy. Now the prosecution just looks like it has so little it has to resort to deliberate confusion.