The way I think of it, once Platinum Angel extends its protection to you, you are deathproof, but not damage-proof. You are not immune to bodily harm, but you are immune to dying from that harm. If you win a duel at negative life, you’re a messy pulp, but a very grateful messy pulp with the handy inability to die. Then if Platinum Angel goes away before you have time to recuperate, you’re just the pulp. Flatline noise. Time to get a body bag and a broom.