We really don't know much about the real Arthur, but if he was real, he wasn't a king but rather a fourth-century warrior who was a hero to the Britons. As time goes by, heroes get elevated, in this case to a king and a legend. The story of Arthur was written and re-written over centuries to include other well-known legends, such as that of the round table, the knights who each had their own powers like MCU characters, the Holy Grail, the affair between Lancelot and Guinevere, and most of all, that sword. There are actually two swords, Excalibur and Caliburn, one from a stone and the other from the Lady in the Lake, even though strange women lying in ponds distributing swords is no basis for a system of government. All of these stories may have come from some grain of truth that turned into a good story and then became folded into the Arthurian saga as it was resurrected again and again. Arthurian legends are like Katamari Damacy, the game where the further you roll, the more stuff you pick up along the way. You can't keep a good story down.