Went back and viewed Martin's video again and I remembered the issue incorrectly, Martin understands the basic nature of this valve. The problem is that instead of demonstrating the Bruno Is. accident, he speculates that it had to be a very low power shot. He asserts this on the fact that with his gun the air pressure has to go down to 300-400 psi before the hammer spring along will open the valve and the gun will discharge. (So, he fully understands the issue involved) Martin does not state what velocity the .41 cal bullet is propelled at with 300-400 psi other than to say it would be very low.
Martin goes on to demonstrate proper management when no decocking lever is present by backing off the tank.
With the Bruno Is. incident, however, there is no reason to assume that only the hammer spring pressure would be applied. Instead, since the person holding the airgun was not familiar with it's management, we must assume that the shooter only knew how to safely manage a flintlock rifle. To put a flintlock hammer down in safe decock position you normally move the hammer slowly but firmly all the way down. So, at Bruno Is., the hammer is not being gently rested on the exhaust valve (this guy doesn't even know what an exhaust valve is) instead it is being firmly pushed down. It is also possible that the hammer was resting on the exhaust valve and it was bumped.
So, my apologies to Martin, I'm getting old and didn't exactly remember the issue.