There's gotta be a SHO guy in the area that can have a look at it. If it's a single broken piston ring, it should still start and run...it would make quite a racket at idle, but I'd think it would start up, no problem.
I've definitely started these SHOs with more than 1 plug wire diconnected. I've even started one missing a spark plug before. One broken piston will not stop the combustion process in the other 5 holes.
It should take a shop about 10 minutes to pull the intake and another 10 minutes pop off the valve covers so that the valvetrain can be inspected. My guess is that they did a compression test, saw the low number and decided they'd rather not mess with it so they named the worst possible cause.
Tell them that you think there's a shim that popped out of it's bucket and it's probably laying in the head.
A cracked piston ring would cause the charge to go into the crank case, where it would escape through the crank case ventilation system.
A spit shim is probably the result of a valve that got stuck open. If it was an intake valve, the charge would be shot back into the intake where it could cause all kinds of havoc with the combustion process of the other 5 holes.
I guess it doesn't matter...if you've got a stuck valve and a spit shim, or a cracked piston, the engine should be replaced.