Why is this impossible?

Rob Dreher writes:

Unless forensics comes back with some smoking-gun evidence, it’s hard to see at this moment that Zimmerman’s guilt could be proven beyond a reasonable doubt.

This case may turn on poor police forensics work on the night of the shooting. But that’s not George Zimmerman’s fault.

What evidence do we have (evidence, not the self-serving statements of lawyers) that the forensics work is poor? Or that the police investigation was slipshod?

Why is it inconceivable that the police work and the forensics evidence confirm the basics of Zimmerman's story?

I'm not saying that is the case, but isn't it possibile that Zimmerman is telling the truth?