It doesn't really take away from his guilt if it wasn't premeditated, the only difference is it could spare him the death penalty if that's what the jurors conclude.
I think you must be referring to Geragos' statement in his opening that Laci had seen the boat over at Scott's warehouse. Geragos says this will come out.
I'd be wary of promises from Geragos, though. I understand he often fails to deliver. We'll see if he really produces a witness who says they saw Laci going in and out of Scott's warehouse when the boat was in there. I kind of doubt he will.