By that logic, since Paladin was more than capable of clearing the entirety of Alexander during Heavensward, it shouldn't have changed one iota in Stormblood because it didn't need them.
The thing is, Dark Knight DO manage it, they DO manage all the content right now, but have some unique challenges that the other two tanks already have answers for.

I haven't mentioned this much in threads, but I don't see it said often enough and I'd like to assert this factor. I often found myself wondering why I took Dark Knight a little more personally than I had my other main jobs throughout my time playing FFXIV. People laughed at me on Dragoon in early 2.x, I didn't care. I trudged through Machinist during the early days of Heavensward with pride. Why Dark Knight then, has me so concerned?

It is because, even beyond healers, Tanks take responsibility for every overall fight. A badly performing tank is magnified across all levels of the fight and affects everybody. Praying for my TBN to line up correctly while my other cooldowns are zilch, hoping I'm not too much of a distraction by needing to be healed constantly between cooldowns, feeling the guilt inherent in Living Dead forcing healers to stop what they're doing and pool resources they may have not had to use otherwise on me, etc. etc.
In Heavensward Dark Knight had an answer to many of these areas of critical contention in performance, yet in Stormblood it often feels as though I'm hindering my party instead.

So I feel the passion and anger and sadness many Dark Knights suffer is this inferiority in accepting the responsibility of the role that is the standard. These little issues magnify and add up exponentially on the tanking role.

So yes, Dark Knights do manage currently. And feel the chains of guilt knowing that they held their party back just that little bit, which matters even moreso on tanks. (Admittedly that sounds like a very Dark Knight thing to feel. Maybe we're just saving up for a major Minus Strike for 5.0.)