It's a bit of a stretch to say "psychic powers."
Great Swords are one of the most popular and iconic weapons of fantasy games.
Dark Knights have been in almost every FF game with jobs (and a few without) since jobs were a thing.
Thus, it stands to reason that DRK would be one of the first jobs added. It was either that or Samurai by SE's own admission.
Now, what *I* find funny is that it's not going to be possible for the DRK to do what is shown in the trailer, since it's the melee DPS limit break.
So yes, in all irony the great sword wielding class won't be able to use the limit break with the sword.
Though they've said they're overhauling the LB system soooooo