> 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.
This is super important to emphasize. In its first hundred years, the printing industry made much (if not most) of its profit off of what were essentially trashy paperbacks, pumped out as fast as they could acquire them because there was no concept of copyright yet. The authors tasked with writing that content took all the old history books - most of dubious historicity to begin with - and wrote wild fantasies based around them. The King Arthur legend is one of the most popular ones, probably as widely read as the flood of translations of the classics.
Since this coincided with a large increase in literacy, these fiction books calcified the fiction into legend and pop culture.