No one understood the boundary between life and death better than Tyrion and his undead sailors.
The death god, Bartok, had a door that anchored the boundary between life and death; in short, as long as the soul of the living passed through that door, it would arrive in the world of the dead, and that door was one-way—in other words, unless one passed through that door, whether it was the brief revival of a corpse or the sustained curse of the undead, it wasn't considered true "resurrection."
"Many people in the world confuse 'undead' with 'dead,' even thinking the former are taking advantage of a loophole in the door of the death god," Tyrion turned around, looked at the noisy sailors in the square, and said calmly, "But in fact, they are just souls rejected by that door due to corruption, thus stuck at the border between life and death. If we go by the strict definitions of the Death Church, 'undead' actually belongs to the world of the living."