They obviously aren’t in Purgatory. Unless somehow Purgatory is accessible via helicopter and submarine. There is obviously some reason why parachute-gal thinks everyone on 815 is dead but we just don’t know what it is.
Remember, the tide came in and washed the fuselage off the beach (and the tail section landed off shore in the first place). Maybe the wreckage somehow got carried by the current to somewhere that it could be found, the searchers found it and the bodies inside and assumed everyone had died. Given that the interior of the fuselage was burned, so they may not have been able to determine how many bodies there were and they may also have assumed a fire on board was responsible for the crash.
I believe that the “black box” is usually in the tail so they may even have found it.