You didn't read the math. DRK takes more damage from even magic damage than GNB and PLD over a full rotation of defensive cooldowns. The DRK falls behind in all defensive fronts. And before I hear anything else about stacking defensives with TBN, the fact that Dark Mind is 20% instead of Camo's 10% or Bulwarks 16%, after diminishing returns from those stacks the difference is in the single digits for the other tanks that also get greater ease of access to their 25s cooldowns. So maybe DRK can stack 20% with 20% with 10%, GNB PLD Without using their 90s cooldowns can do 20% with 15% with 15% and hit themselves with some health, and when tanking, the only thing that matters is the state of the tank after the damage has been received meaning if all tanks survive then the only thing that matters is how much health they have afterwards, and since PLD, WAR, and GNB can restore, they end the damage at a healthier quantity of health. TBN is only better if it would have outright killed you and I've been proving this for years. Shielding and healing have different functions being post-damage restored health and pre-damage protective health, but both are health that benefit from damage reductions all the same. The numbers are there, the logic is there. TBN just has the greater potential, but nothing to apply itself well enough to actually be any good next to the other tanks.
15%+15%+20% damage mitigation is 42.2% reduction for GNB and PLD. DRK gets 20%+20%+10% is 42.4% after diminishing returns, GNB and PLD making a difference of just .2% for even magic damage but with 1 less cooldown and their own cooldowns having better uptimes and effects. The only difference is TBN gives you extra eHP before damage and the others get it after, but the value they get after is greater than TBN offers.
All tanks in this game are ridiculous, content is basically catered to us, but DRK is the worst one and not by a small margin which makes it feel bad. Do Expert for 2 weeks on everything but DRK, then go back in with DRK in the same ilvl and you'll begin to see what I mean. DRK can do it all just fine, but the other tanks do it significantly better.