I think once a lawsuit is lost by a gunmaker, that can then proceed to the SCOTUS. The only reason one should be able to sue a manufacturer is for a manufacturing defect. If the gun used shot straight and reliably, there was no manufacturing defect.
Exactly, and it's a shame that the article didn't explain that. Having the Supreme Court step in before a trial/verdict would have been very unusual procedurally, and I suspect that's why review was denied.