Well, first of all, no. Tank barriers scale off of Attack Power, not Max HP, unless the barrier specifically states that it is based on Max HP. So, Veil, TBN, Shake, are based on Max HP, because they state as much. The barrier effect of Guardian (1000p) is based on 'an amount equal to this tank doing a 1000p heal on themselves'. Coincidentally, PLD has access to Clemency, a 1000p heal, so it's pretty easy to see how much 1000p would actually be for them in any given gear.
Secondly, Barrier effects have a duration. The times between CDs (and there will be times), are the points where the Tanks take damage. But unlike currently, the damage won't be healed back by Bloodwhetting and the like. Once the HP is lost, it's lost until the Healer heals it back.
I dunno what they were writing about, as their numbers would be a nerf to the overall sustain of the Tanks. I don't think the Tanks need their sustain 'nerfed' per se, just tweaked so that the effect of the sustain is a little more... temporary. Plus, it'd introduce a new kind of 'skill': being able to manage your selfsustain effectively, to have higher coverage of barriers to mitigate
Its the combination of 'being able to mitigate so much, that the incoming damage is such a small number' in combination with 'the amount of selfhealing the tank brings'. Look again at PLD. Every 22ish sec, you get 1000p of healing for free, via Holy Sheltron, on top of it mitigating for so much. Every 1min, you get, say, 3 Divine Might procs, a Confiteor, and 3 Blades, each giving you 400p of healing when used. So in total, you're getting like... 5800p? of healing, by just playing your job and pressing buttons you'd already be pressing because they're part of your rotation/they've also got mitigative effects. It's not just that your mitigation kit is strong, it's that you've got so much incidental healing coming in supplementing it.
Now, you bring up Guardian. Yes, Guardian is strong. It should be, it's a 2min CD. When it's not available, you're using other mitigations, and healing HP with your actions. When you restore HP, you don't lose it again until the enemy takes it from you. When you press Guardian, you get 1000p worth of 'HP' via the barrier, yes, but at the end of Guardian's duration, you lose that barrier whether it was consumed or not. That's the point of the change: If you play sloppy and just hit whatever, whenever, you'll open up too many holes in your mitigation coverage, and take 'real damage' more often. 'Real damage' that you would no longer be able to simply heal through via Holy Spirits/Confiteor/Blades, or Bloodwhetting, or Aurora.
Out of all the Tanks, DRK is the 'least problematic' when it comes to this topic. People complain about '1DRK 3 DPS' EX roulette runs FAR less than WAR or PLD being in one. Why is that, when DRK's mitigation kit is so damn strong? It's simple: It's got the least selfhealing by far. It's very barrier-focused, with TBN, and when TBN isn't up, it's taking 'real damage' that it can't heal back easily (Abyssal Drain has a 1min CD). DRK is the proof, I'd argue, that such a change would work extremely well.