[Steko]

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.