It absolutely scales off HP. If the enemy doesn't deal damage to me... then great! The enemy isn't dealing damage to me! But the enemy WILL be dealing damage to me. I'm going to be in full tank build and I'm going to be running up to people and being a meat shield. I'm going to be taking damage. Once I hit 2000 hp, I'm going to get 500+ true damage off from my W no problem in 1v1s and team fights.
I mean... what are you going to? Just not hit the Sett when he walks up to trade with you in lane? You're going to have to deal damage to him.