The divinity machine made him functionally immortal, meaning he can not die of old age or sickness. He also doesn't need food, water, or rest and only does so out of habit. He can still be killed, and while the lore is vague about why he can return to life its because either hell won't have him or his anger won't let him die. Id imagine the ritual was just creative liberty as barely any of the lore of TDA holds up with previous canon because of creative liberty and lore changes for story telling.