|“||Lord Beckett has recruited an army of assassins, and there's no parley with Jolly Roger.||”|
At this point, he cannot be harmed. Parley is not considered concluded until said captives and captains have completed negotiations.
Unlike "aggressively negotiating" with an enemy in a boarding party, Parley in this case means the ship is NOT boarded. Instead the enemy vessel sinks instantly and rewards a smaller amount of loot.
The word "parley" is believed to be related to the word "parler", which is French for the English word "Talk".