What I think his statement (ie:“This is the first time in my memory that anyone was ever killed in one of these accidents,”) was meant to mean is accidents in which subs have hit undersea obstructions.
The Thresher disaster was caused by inferior welding in the construction process
Not to mention being blasted by mines for hours where they had miscalculated the actual blast wave due to improperly accounting for the bottom reflection in shallow water.