No, D&D is definitely set in medieval Europe except with magic added, and the mythical creatures are based on European fairy tales (albeit ones before the medieval era). The weapons are also medieval era specifically, with types of swords and armor that didn't exist prior and weren't used later.
Maybe it doesn't represent feudalism, but this was the inspiration, not the USA. And macro economy isn't a focus in the game. If you want to make it fit, you could say the adventurers are not adhering to the system, rather they're rebels, nomads, or pirates.
I think the author's issue is he conflates feudalism, which is generally held to begin in the 10th century, with the entire medieval period, which is traditionally dated as starting in the late 5th century. He also thinks of feudalism as this static culturally defining force but in reality it waxed and waned depending on the time and place.
It also had some huge holes in who and what it covered, and it's not hard to imagine any of the OD&D classes (cleric, magic-user, fighting-man) in those gaps. The largest of these gaps by far was The Church, but we also have universities (which developed under protection of the church), guilds (which developed in places under protection of the universities), and the rising merchant class (who could form guilds to reinforce their power). There were also mercenaries, hermits and various other free people.
It's the whole point of the fine article - that very little of actual medieval Europe is in there. It's only in there as a faint backdrop meant to provide some kind of "comon" image in the mind of the players. Exactly like the early sword and sorcery stories, or Conan comics.
I can see that, but plate armor was prominent in the later middle ages too, pre-Renaissance. And D&D has no gunpowder weapons, which were starting to be common in the Renaissance.
Then again a lot of "medieval" movies/TV/games mix Renaissance stuff in.
the DMG has Pistols & Muskets, and there's also Laser Rifles in at least Rime of the Frostmaiden. They just tend to not get used cause they don't really fit in the rest of the setting.
Maybe it doesn't represent feudalism, but this was the inspiration, not the USA. And macro economy isn't a focus in the game. If you want to make it fit, you could say the adventurers are not adhering to the system, rather they're rebels, nomads, or pirates.