What they mean by "change the damage calculations of tanks"?

[Cynric]

snip.

Shadowskin is 20% same as rampart so no that's wrong.
Shadow wall is indeed weaker by 10% but that's negligible. And only one CD. We can easily time reprisal to replace that using Dark Dance. Reprisal also stacks with Storms path and works on all damage types. We also have a special 15/30% CD on a short cooldown for magic damage specifically that paladin does not have.

So the trade off for having to use reprisal or another minor CD with shadow wall to be equivalent or a little less than sentinel is the ability to also mitigate 15/30%+ more magic damage than a Paladin can?
Paladin's best mitigation tools over Dark Knight are hallowed ground and the ability to cross class stone skin.

And yes paladin can block more damage than parry can, but RNG is still RNG. I would hope they don't balance encounters based on whether a shield block or parry goes off during a tank buster.