Eh, not exactly true. It's just a really high gear check.
For the sake of argument lets say that Mandokir always inflicts the maximum amount of damage with this attack. I'll be using
my DK as an example and running a maxed Blood Shield going into the fight.
1. Weakened Blows reduces damage dealt by 10% so 15,375,000 * (1-0.1) = 13,837,500
XXX2.. In-game as Blood with Blood Presence active I mitigate 52.45% of all Physical damage. That means that before all other damage reductions are considered I only take 13,837,500*(1.00-.5245) = 6,579,731 physical damage.
XXX3. Defensive cooldowns add in a multiplicative fashion so: Damage taken from cds = 6,579,731*[Bone Shield's damage taken (1-0.2) * IBF's damage taken (1-0.5) * AotD's damage taken with full procs (1-0.5208)**] = 1,261203 Physical damage taken
XXX4. Blood Shield caps at your max life so 1,261203 - 532,999 = 728,204 damage
XXX5. Damage from 4 - my life = 195,205 (overkill)
As we can see my DK would not survive the attack and thus not benefit from the Vengeance of that attack due to the removal of overkill Vengeance. However I am not in fully optimized gear either. Best gear I imagine is Parry+Dodge ASSUMING same Strength values as on dps pieces. Trinkets I'd want either the Shado-Pan Assault tank trinket OR Relic of Niuzao + Stuff of Nightmares OR Fortitude of the Zandalari. If someone wants to run the numbers of a Blood DK in full ToT LFR gear be my guest.
**Assuming my trinkets+FC+DRW are up so that my Parry+Dodge = Relic+Lei Shin's+FC+DRW+Dodge = 47.08% (Parry) + 5.46% (Dodge) = 52.08%
XXX Edit: Forgot to account for Weakened Blows. Reworking the napkin math now.