Armour has effect on the damage you take, since he still does AD with his basic attacks. The problem lies within the 4% Max HP damage with each attack and the true damage stacking with his passive. He gets about, at max effect, 30 true damage/second, which lasts for about 6 seconds, iirc. So he'll be doing 180 true damage in total with his passive, which isn't too much.
However, to stack that, he has to attack 6 times, which is 24% of your max health, assuming he has Bloodrazer, so you're already at 3/4s of your total health, not including the damage his basic attacks do, which, if he has Infinity Edge, would do 173 damage, without any armor. With standard armor, people have about 70 - 80 armor by natural scaling, so you'd get about 59% damage reduction, so 173*59% = 102.07, so you took about 612 damage from basic attacks.
Right there, you're down by 1/4 health, having been dealt 612 AD and 180 true damage. Then comes the expunge. Expunge (or Contaminate as it's known now) does 80 base damage + 35 damage per stack, so with 6 stacks, you'll take 290 damage without scaling damage. The damage as a whole doesn't scale, it's a per-stack scaling. So, with 173 AD, scaling by 25% with each stack, you get 43.25, which we round down to 43. That's an extra 260 damage, so his E does 550 damage overall, not counting armor. (If you counted armor, you'd only be dealt 150 AD in scaling.)
So, 612 damage plus 550 damage from expunge is 1172 damage, plus his passive 180 brings you to a grand total of 1352. His passive dealt about 13% of the total damage he did to a single target. So, his damage isn't dealt primarily through his passive, no. It just gives a nice amount of damage to supplement the rest of his damage.
(This is all without reducing the Bloodrazer damage due to armour. If you want an accurate measurement of how quickly he killed OP, we need to know the details of OP's match)
