One thing that the grim/dark fantasy article kind of glosses over (though you don't in response to rysmiel) is the degree to which the grimdark trend of the last decade or so is a deconstructive response to the more straightforward (and often problematic) epic fantasies of the '80s and '90s and before. Whether this is true on an individual author level is basically irrelevant, because even if the likes of Mr. Lawrence haven't read any of prior art in their subgenre, one can be certain that at least some of the agents, editors, and other publishing professionals involved in acquiring and selling their book are aware of it.

(There's also the argument that a fair number of the authors producing work in said sub-category grew up reading the likes of Watchmen and Dark Knight Returns or the wave of imitators that followed them, in much the same way that the popularity of Vampire: the Masquerade, LARPing, and other World of Darkness stuff in the '90s probably played a role in the rise of Urban Fantasy. Even people who insist vocally that they are totally original dammit tend to absorb influences from the zeitgeist in one way or another.)