Shooting at aircrew who've had to bail out is a war crime, as well as being despicable.
One wartime Luftwaffe commander told his pilots "If any of you shoot at an enemy pilot in his parachute, I will personally shoot you myself.....and I won't miss!"
If the pilot in the OP's link managed to hit the Zero pilot, then it was an impressive feat of marksmanship (or a lucky one), to be sure. But just because the average pilot's skill with a handgun is less than his being able to hit a cow's arse with a banjo, there's no reason to discredit the story.