As Brian says, the technical issues of your description may be easy to achieve as in Maine in terms of quality of materials and mechanics. This isn't the rudder of a power boat, for example.

Design of the components and fabrication specs are not so simple. Depending on the design of that rudder (which failed already), the design really isn't trivial. First, a hydrodynamic loading with moveable control surface is really dependent on complicated design with some safety factors "thrown in". With forces proportional to exponential differences, say a 25% difference can be very material. Then there are the issues of a composite structure, where the interaction of the components is very different depending on quality, tolerances, etc.

In this case, as I and others have conjectured, it appears to me the it was more than the rudder that became issues. The modified skeg rudder didn't just disappear. The lower pin had to fail again or was otherwise damaged, and remained in adequately addressed. That pin and assembly along with it's imbedment is a non- trivial issue to deal with.

Most repair locations, including those in Maine, address this issues using experience rather than engineering. In this case, it isn't clear what the repair shop did. Did it just try to copy the original factory construction, or not?

In this case, it is easy to conclude (assuming that the boat didn't hit something [else]) that the repair was not adequate. Whether the original design for the boat was really adequate seems something harder to suggest because of a lack of reports of similar failures.

I still wonder what the meant by "totally gone"? What hole did it leave (in the tube, if nothing else)?

Shit happens. I'm not sure I would cast condemn that couple for calling for assistance and the taking the direction to "get off the bus, Gus".
The VAST majority of thos reading this wouldn't have gotten on a small sailboat in the winter and made that journey without mechanical problems. However, the observation that someone (as others have shown) might have done otherwise is both true and informative.