I very much understand the Court of Appeal's ruling. It's a death penalty case, and an automatic appeal is accordingly pending before the California Supreme Court. So the Court of Appeal holds that the trial court does not have jurisdiction over a resentencing petition, since that might interfere with the disposition of the automatic appeal in the California Supremes.
Though, as the opinion notes, the murders here transpired in the previous century, the death sentences at issue were imposed a quarter century ago, the briefing in the California Supreme Court took seventeen years, and the California Supreme Court has been "working" on that appeal for the past eight full years.
Given these facts, as a practical matter, I'm not sure that there's a ton of hard work going on with which to interfere anyway.
But I get it. Doctrine. Exclusive jurisdiction. Understood.
