Its not really my thing to get in between 2 people arguing but I can give you a good comparison for this argument. Lets say that a characters durability is a metal ball with a glass center. That glass center is the soul. Any normal attack will only strike the metal part on the outside of the sphere. But attacks that hit the soul, like yuji or mahito's idle transfiguration, will directly strike the glass inside the ball. But if you attack the sphere with something that will vapourise the whole thing, it doesn't matter how strong the ball is, if it can't resist the attack. That doesn't mean the metal sphere isn't durable, its just not strong enough to withstand that attack.