Yeah, and also while DRK did wear metal armor, it was never really heavy armor. Just because its called a knight does not mean anything. I mean look at batman, he was called "The Dark Knight" and he wore very flexible armor in the recent films.
Also you need to look at not just 11 but also tactics and every other incarnation of the class. Look at wiki, the DRK is supposed to be an awesome damage dealing class compared to none, usually the most damaging class in the game due to its willingness to hurt itself to kill its
opponant. While some goofs as you mentioned are there, I too have to agree the DRK is going to need to be a damage dealer.
Souce:
Wiki Dark Knight