I'm not sure if the bolded part is true. Tankbusters in this game can be tuned to almost 300% of a tank's maximum HP, unmitigated. It's difficult to imagine a system of flat-HP mitigation skills that would allow DRK to survive something like Soul Tether, even if it's married to something like Rampart which remains percentage-based as a role skill. It's more difficult still to imagine a system that would allow DRK to survive it without effectively giving DRK the ability to completely nullify the majority of "normal" Savage-sized (80~100% of max HP, unmitigated) tankbusters.
In general, flat-value mitigation is a bit tricky to balance, because at certain ranges of incoming damage, it very quickly becomes overpowered, but as the damage scales up, it pretty quickly goes from "average" to woefully underpowered. In FFXIV, I don't think you could build a tank around the concept that relies on it much more than DRK already does without creating a situation where certain things are needlessly easy and others are overly difficult to deal with.
