You can barely call D&D anti-medieval; it isn't from a world of obsessing about Tolkien-style fantasy. It's Gygax coming up with rules for miniatures wargaming where players are individuals within a group rather than being entire sides of a war and moving armies, or being squad-level and choosing how to move squad members. That was the important part that influenced the entire world. All of the players were part of a single squad, and working (and cooperating) as individuals for their own benefit.
These rules were then applied to Gygax's (and everybody else's) favorite fantasy novels. The thing that varied most about those novels was the idea of magic, so the only influence on his system from fiction that I recall is the stat-friendly Jack Vance magic, which would end up imposed onto other settings.
But it's still fair to call the system anti-medieval as the article does because it was made for a competitive multiplayer tabletop game which was meant to progress over sessions, and the main aspect of its progression are stats. So it has to be as fair as possible, and you have to be able to accumulate indefinitely rather than die in the same place you were born with no more than your parents had. That's American myth, not medieval reality. There can't be a medieval system, because that would crush all of the characters, starting by burning all of the witches. If every character were a fighter trying to get ahead by fighting in an army, there's no D&D, because D&D is individual, not squad or army.