Just because you say it doesn't make it true. So far all anyone has proved is that DRK is a little better at magic mitigation. But certainly not on the level that PLD is better at physical. With other classes, their role is big an obvious. For DRK people struggle to find areas that they might be slightly better at. Nor are they better at defensive team support with their pety, rng based parry proc that can't be relied upon for important moments, and can only be used on bosses they are MTing (and ones that have a steady parrage of parriable attacks, so say, Ramuh, would never be reprisaled, making their whole "magic damage tank" shtick a hell of a lot weaker).