I don't think the comment has anything to do with Dark Knight being a tank as opposed to a DPS, it's more saying something along the lines of Dark Knight is just more of the same. But honestly it's entirely possible Dark Knight will be a kind of mage tank. It'd be a kind of different take on it, but I could see them doing that as a means of changing things up a bit, and it'd still be fitting for Dark Knight.

And single player FF games have plenty of jobs that work conceptually as tanks. Dark Knight is one of those, that people want to complain about it is just people ignoring the qualities it has that make it tank-like. As well, while every FF game that has a job system tends to add a few new jobs here and there, the majority are typically recurring jobs. As things stand now we have a very small subsection of jobs that are pretty decently recurring. There isn't anything wrong with them continuing to add old jobs, since there is still a lot of jobs they can add that work as tanks, and to a lesser extent healers.