The dating is reinforced by the lack of stirrups. Rome's first encounter with the stirrup was the type used by the Sarmatians, who, after their defeat by Rome, were parted out to different parts of the Empire, with the Sarmatian horsemen formed into auxiliaries for service in places like (ta-dah) Britain. There's a sort of eccentric view that the "real King Arthur" was a Sarmatian cavalry commander. Stirrups probably originated all over the place; the earliest representation in ancient art is from India.
I don’t know how old the stirrup idea actually is... BUT I thought that the innovation didn’t reach Europe until after the first sack of Rome. Was it 437 AD?
About 60 years ago, while in Uncle Sam’s Army in Japan, I checked out a library book, THE SECRET OF THE HITTITES, of all things. The author, of whose name I have no clue, made a number of claims as to the forgotten power and influence the Hittites exerted upon surrounding cultures, including, if memory serves, ancient Egypt. One of which was that they were one of (the first?) to attach stirrups to their saddles. FWIW.
Interesting theory about the origin of the Arthur legend.