It's mainly really, really bad history

> Each lord had a veto power over the king and over each other law (I will use the term “lord” for those landed free men. Even the serfs could not be denied their right without adjudication. Land was not held as a favor from the king; title was allodial. A man’s home truly was his castle.

This is so bad it's basically a total lie. Let's start with how he does not discern between free, landholding men, minor nobility and great aristocrats, three classes of men who would have absolutely no problem realizing that their lives, political power and the law that actually was applied to them were very different from each other.

The vast majority of men living in the middle ages did NOT actually have a fortified private manor aka castle, but a hovel that was protected from violation by other people most probably by the local lord or gentry, who as it happens was the biggest danger to your freedom as well as the judge over what rights you had.

>It seems, instead of the pinnacle of governance and protection of liberty, the constitutional form represents a significant step back from the liberties afforded to even the lowliest members of early mediaeval society.

That's not a point, that's straight up pure fantasy. The lowliest member of mediaeval society, unfree people, had more rights than we do now the same way that chattel slaves in the American South had more freedom then than as black people today, shoulders brutally weighed down with responsibility for their own life instead of a simple pleasure of having a master take on that dastardly responsibility.

The author has cherry picked data points, badly misunderstood concepts used in those cherry picked points, and then wildly conflated vaguely related things to create a utopian version of his entirely unhistorical mediaeval heaven.