The question then lies in how "tanky" does something have to be in a game that doesn't actually have any tanks in order to qualify as a tank in a game that does? It's part of the problem with trying to assign roles to classes that existed in games that didn't have roles.
As I see it, the differentiation can't be determined based upon offensive capability, mainly because *every* character in an FF game is general designed around being able to dole out damage as effectively as possible, with the possible exception of your healer. In the pre-MMO FF games, there isn't a "tank" role: there's DPS and healers. Even Cecil, the fundamental example of an FF Paladin was able to hold his own in the beating down of foes. Using the offensive capability of a character from those games to define their functionality in a game that actually *has* defined roles. It's for *this* reason that I see Dark Knight as being a tank: it was always "tanky". It always had high defenses. The method in which it dealt damage differed from others (essentially, stabbing things and bringing its own unique brand of magic), but it was still, fundamentally, one of the more durable classes to have around.
I still have to wonder how anyone could possibly envision Ninja as a tank, especially when the base class has pretty much been announced as "Thief". The only time I've ever seen a Ninja or Ninja-like class defined as anything other than a damage dealer was in FFXI. Every other thief or ninja in the entire series was *explicitly* a damage dealer. It would be like saying that a White Mage should be a tank because it can cast Protect.More than likely, the next tank class will be Ninja.
The idea that Ninja will be a tank in FFXIV is simply dumbfounding, especially since the only reason that anyone *thinks* it will be a tank is because of how players applied the abilities given to it in a game without well defined roles without regard for intent (Red Mage was supposed to be a hybrid and instead became a buff bot thanks to a small number of spells). FFXIV has explicitly brought in classes with their abilities and role attached to a given intent. I would be willing to be *anything* that Ninja is going to be a DPS job, not a tank job. Having it be a tank job is just ludicrous.