Do we suppose that the force of the wave snapped the tether? That would mean the force of the water on his body was 2000-4000 pounds, equivalent to strapping a 150 hp engine to his butt and blasting him through the water at ridiculous speed. Improbable. The force on his body was probably more in line with a serious wave break, perhaps 500-1000 pounds.
Do we suppose that the impact of his body against the anchor point, without any stretch in the tether, contributed a major part of the force? More probable. Flying across the deck at 8-10 knots could account for all the force.
Sail Delmarva: Dynamic Tethers
A combination? Certainly. However, it seems quite probably that tethers with a little give in them could prevent this sort of breakage. Making them even stronger probably won't help; if the forces on the body are more than 5000 pounds the impact would be fatal.