Yes, "very strong piano hauler". Actually, I don't think he is the cause of the failure. Getting a wrench on the lower nut and being able to resist the amount of torque imparted to the top nut that would fracture that stud just doesn't compute for me. I see the stud as being the strongest part of the whole system, unless it had a problem. I think the stud had a failure; I just don't describe it exactly the same way that Yanmar did.
I have had further thoughts regarding the mount configuration. The rubber mounts probably serve as vibration isolators; my solution would work just fine, except that it could transmit vibration into the mount pads in the boat. In any respect, there should be no reason for you to ever have to do anything further. I just don't like the idea of using threaded areas for structural purposes. They're great for imparting tension into the fastener. Beyond that, they're a liability.