Nope. I don't "know" that. In fact I don't think that's true at all. And Martin wasn't sitting on Zimmerman's knees. Martin's knees were on Zimmerman's arms according to Zimmerman. Knees on arms, butt on chest. If Zimmerman "weaseled his arm free" he still would have Martin sitting on the gun. At this point Martin would probably have felt the gun and would know he didn't want Zimmerman getting to it. Oh yeah, and Zimmerman's getting punched in the face the whole time. From my own submission wrestling experience, and talking to others in self defense, I honestly don't see how Zimmerman got to gun without first getting Martin off of him. Now I can easily see how Zimmerman could get Martin off of him in this situation. It's a simple matter of bump and roll. Considering Zimmerman outweighed Martin by 40 to 60 lbs that should have been relatively easy.
The simplest explanation is that either Zimmerman threw Martin off of him outright or bumped and rolled into Martin's guard. I'm not saying Zimmerman was standing up. Just that he was most likely on top of Martin when the shot occurred. And yes. It matters quite a bit how Zimmerman got to his gun. Because if he established a dominant position first, that reduces the veracity of the self defense claim.