partly true, and partly not true. I think it should be explained a bit further.
Busters that do more than max HP in damage, unmitigated, require a limited "type" of mitigation. (literal mitigation) fluff mitigation doesnt work on it.
Essentially you can have fluff mitigation be more useful, and the more "normal" type, which tanks need to maintain, and the TB styled mitigation being only there solely for tank busters.
Example: Convalescence, Anticipation, Blood Bath, Equilibrium, Clemency, bulwark, featherfoot, Flash, etc were all methods in which to make life easier for a healer, but werent good enough to survive a tank buster.
Mitigation for tank busters are a form of -X%(rampart/hallowed), +tempHP (Thrill/TBN), cant die (Holmgang/living dead), and "conditional" (Awareness/Stun/Silence)
These can save a tank from an attack that would do maximum HP dmg to the tank.
In short, its possible to make enough fluff mitigation to make life easier on healers, and increasing auto attack dmg from bosses.
Its also possible to up tank buster damage, and tank buster mitigation, so long as its limited for tank busters only. (short duration, and not always available)
This creates a dynamic of changing up how you mitigate dmg through out every encounter. BUT it also limits the methods in which you can base a tank thematically. (Since now all tanks require 2 types of mitigation, and making those 2 types unique amoung all the tanks would either limit your options, or create overlap in mitigation design. But to be fair, there's FAR more stypes of "literal mitigation" thats either never been done in MMOs, or hardly done in MMOs, which FFXIV can still tap into. Fluff mitigation is far more limiting. (Another option is the -5% and -10% mitigation being used for fluff, as long as all tank busters do at LEAST 15%+ more dmg than the tank has in HP, to make sure those options arent enough for busters)