Unless the tether actually failed, or the attachment point failed. It does not really go into detail, my guess would be that he had to reach something that was out of reach of the tether when trying to get the downed mizzen mast off the boat and disconnected the tether. A real shame.
Everything has a load limit.
I remember reading that the hook on someone's teather straightened out under load.
In this test, a harness was placed on a 220 pound dummy. Each tether to be tested was attached
to the harness and the dummy was raised up to a quick release shackle such that when the
shackle was released, the dummy would free fall 6.6 feet. Failure criteria include "flaws, defects,
or deterioration after testing that would jeopardize the safety of the wearer".
General Comments on the Tethers:
We were somewhat surprised that there were so many tether failures. 47% of the tethers failed in
such a way as to endanger the wearer. Failures were both in the hardware, stitching, or
sometimes both. Build quality of the tethers varied considerably from company to company.
This was based on only a 6' free fall.
It is easy to imagine conditions that could exceed that by several times.
For example a wave grabs you and instead of having only 300 lbs on your tether you have, including water weight 1,000 lbs or more.
So yea tether is great, probably the best we have but the forces that hit you can exceed the strength of anything you would be willing to wear.
Also assuming you did remain attached to the boat during an extreem encounter what are the chances that all of your bones will be be intact.