Perhaps I'm oversimplifying things, but I have personally always assumed that using "Medieval" to describe D&D was almost entirely a reflection of the (non-magical) level of technology, and not anything societal or cultural.
> The D&D weapon list has a medieval feel to it, but partly that’s just because that’s what we’re expecting to find. In fact, it’s a sort of survey of (mostly) pre-gunpowder weapons. Most of the weapons and armor appear in ancient Europe and in Asia as well as in medieval Europe. Partial exceptions: Composite bows are mostly non-European, while longbows are associated with Europe. The halberd is basically a Renaissance weapon, and the two-handed sword appears in medieval Europe, India, and Japan, but not the ancient world. No one knows what “plate mail” is supposed to be.
Gygax himself didn't lean into it as hard, but there were plenty of fairly gonzo modules that did include aspects of future and/or alien technology.
The argument the author is making is messy because it mashes together the dichotomy of old school D&D versions vs. the modern equivalents. Originally, D&D was a framework that you built on top of. You might adhere fairly closely to the rules as written, but they were intentionally quite basic in nature, and additional structure and systems almost always came as part of your campaign milieu and table-specific needs. Modern D&D has significantly more rules - it's a more complete game, with all the good and bad that entails. Neither have a mechanism to force you to use any specific portion of them, and many people don't.
I agree with the sentiment that D&D is not pro-medieval, though depending on which rulebooks you use it might incorporate aspects of medieval society and structure. I also don't think it's anti-medieval, because that implies structure that isn't there. Any given campaign or table might be more or less medieval than any other and still be totally authentic D&D.