The stories around King Arthur are not confined in that way. Heck, Lancelot was a self-insert hero, that came during a renaissance around the myths. He came a few hundred years after the mythos was established.
The historians try to point out that things are as about in-flux as possible, but the author misses it. They do end on a line reflecting the meaning, and explore some of the various aspects, but... they tend to use "is", forgetting that Arthur has been the fanfic of the world since Medieval times, over and over, and over again. You can be certain of exactly nothing when it comes to those myths.
Excalibur? Not in the earliest stories. No sword in stone, no Lady in the Lake. Caliburnus and Excalibur might be the same sword, in which case Caliburn is noted in an early piece of Celtic origin, but just as a sword belonging to a warlord (Bram). Nothing much special about it. (Wikipedia's page on the sword is... Painful. Most of the focus is on the latter adaptions, of course. But the origin is not clearly Welsh. There is... Debate.)
Merlin? There's a possibility that Merlin and Arthur were the same person at one point. Artr Myrrdin Gwyls is one of the earlier mythical names for the magical bard. (Where Robert Jordan pulled a lot of his inspiration for Artur Hawkwing, rather than more modern tales.)
Guinevere? Didn't exist until seven hundred years after the original tales became widespread. Part of the romantic movement.
Arthur fought the Saxons, right? Sometimes it's the Saxons. Sometimes the Romans. Sometimes the French. Whoever the writer felt it in-vogue to fight, pretty much.
Rant over... They got close to the point, but missed it. Arthur is a story without grounding. It's supposed to be something you can take, shape, and tell a moral about your own culture. And we do. The TV shows, movies, and constant reinventions, are what the character has been for most of its existence. A tale, waiting to be told.
A tale that many of us will read, and watch, and listen to, time and time again, for when the next author finds something to be inspired for.