Well the quote
The power of the wave ripped 55 year old Blackman overboard, even though he was attached to the yacht by his safety harness. He was not seen again.
Does not really make sense. I could see being injured, and even drowned if a huge wave hit you while tethered. 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.