What could DRK have learned from FFXI?

[Ultimatecalibur]

Yes. The job with terrible magic defense, an extremely limited assortment of equippable items, and an ability that burns a significant percentage of its own health to deal damage is a tank. The only tank-like thing it had was high armor for that point in the game, and even that would start to fall behind if you for some reason decided to grind out levels for the few reasonably durable characters you get during Cecil's stint as a dark knight.

Sorry but heavy armor and higher hp is pretty much what made characters tanks in those days. Pretty much every character that would be considered a "tank" in the current time would have superior hp and physical defense but have vulnerability to magic. (Spellcasters could almost always no sell any spell they were hit with even with their limited amounts of hp).

Cecil was a melee dps.

Tanks just happen to be a subtype of melee dps with frontal positionals.

Pretty much. There were only 2 real types of characters in those days: Frontliners/Vanguards and Backliners/Rearguards. Vanguards were pretty much expected to take hits while dealing damage with low/no resource cost while Rearguards were pretty much expected to either heal or deal massive damage by expending resources.