Hearing can be just as important as seeing, especially when one of the points of the defense's story relates to the threatening statements that Martin is supposed to have made. I think it's pretty clear that Zimmerman had no idea Martin was on a telephone call when he put together certain elements of his story. For example, it would be like telling your mom the fight with your brother started because he called you a bad name, not knowing that she overheard the whole thing and knows you're the one who did the name-calling. She may not see which of you threw the first punch, but she knows you're not as innocent as you're trying to pretend. JMO.
The complete statement from the article you linked says:
I'm not even sure that's an accurate analysis of the testimony, though. For one thing, Gilbreath stated that Zimmerman still continued to follow:
And during cross from O'Mara, he gives a list of the evidence he knows of that indicates Zimmerman continued to follow:
To me, if you actually look at what is being said, Gilbreath doesn't say they don't have any evidence. In fact, he gives a list of evidence. O'Mara just plays cutesy with it, like, oh, aside from all that evidence you just named, do you have any OTHER evidence? I guess if someone weren't paying attention they could be left with the impression that there wasn't any evidence, but that doesn't appear to be true.
Anyway, that's just how I feel about it.