Well, I'm pretty sure that would be true, if it wasn't for the fact that damage reduction stacks multiplicatively. First, damage reduction from armor and magic resist is calculated. Then, whatever remains of damage after that will be calculated with for instance Warwick or Galio's damage reduction abilities.
So basically, if your armor gave 10% physical damage reduction, a 100 damage ability would deal 90 damage. But Warwick would have his E up, giving him 55% damage reduction. It wouldn't give him 65% damage reduction. It would give 10% of 100, + 55% of 90 = 10+49.5 = 59.5% damage reduction. Which means, the more damage reduction you have built into your kit, the less effective the new Locket active is going to be.
45% is the remaining damage people can do to Warwick after resistances and penetration are calculated. Locket will then calculate 35% of the remaining 45% damage, which will be way lower difference than someone else who doesn't have an ability like that. Not to say it will be useless, but it will definitely not be as good as on other tank-type champions.
I think the biggest abuser of this item would actually be a zone-type champion like Illaoi, who if you dive on her, she can bust out a million damage within the next 2-5 seconds. With this item, she will also take 35% less damage for the full duration of her ultimate, which forces you away from her.