the point that Dershowitz is making as have many, many others is that this WAS a classic case of self defense and he brings to light how the prosecution withheld evidence. this was a POLITICAL trial starting with pressure from the POTUS.
Edit: i understand that Zimmerman was told not to get out of his car in the first place. he did, but then agreed to get back into it. that was when Trayvon attacked him from behind.