This the same "legend" that features a dude whose magical power is being able to stand on one leg for a whole day, and a giant whose dropping eyebrows are held up with pitchforks. Still, an interesting idea.
Magnus Maximus was a Roman figure in the fourth century CE. It is possible that he was the governor of Britain. In 383, his troops proclaimed him emperor. Maximus then invaded Gaul and conquered the Western Roman Empire. Since at least one version of the legend of King Arthur’s European conquests evidently comes from tales of Magnus Maximus, it stands to reason that the similar tale in Culhwch and Olwen has the same origin.
In support of this, the medieval British records reveal a legendary connection between Magnus Maximus and Greece. According to genealogical documents, such as the Harleian MS 3859, Maximus had a son named Anthun. This son appears in various other records as the head of an important dynasty in South Wales. In these records, Anthun is referred to as the “king of Greece.” This is highly significant. Notice that the names Anthun and Arthur are extremely similar. Furthermore, Anthun and Arthur appear to be the only legendary kings of Britain in medieval British records who are also recorded as ruling over Greece.
However, Magnus Maximus did not actually conquer as far as Greece. Yet, his cavalry commander, Andragathius, did conquer in that direction. He led Maximus’ armies as far as Siscia in Croatia in 388. At that time, almost all of the Balkan peninsula was in the administrative territory called the Prefecture of Illyricum. The capital of this prefecture was Thessaloniki in Greece. Therefore, Andragathius did conquer at least part of the territory whose capital was Greece. With these facts in mind, it is very likely that this is the origin of the tradition of Magnus Maximus’ son conquering Greece.
https://greekreporter.com/2024/10/18/legend-king-arthur-conquer-greece/
