Depends on which stories you're referring to, Sir Edward. If you mean de Troyes' works, then yes, it did not fit within that canon. Nor, for Malory, I think. However, all of these Arthurian legends most likely got their start from older Welsh tales; I knew more on this back when I was more interested in Arthur in general. Not that I'm not anymore, just that my interests have moved on and I've only got so much room to keep stuff in my head.
Since they are just stories, the when of it can be of interest too, or rather, a speculative look at what might have been for a given period. I loved Excalibur as a kid, because the knight in shining armor concept that I'd first read about in Bulfinch's mythology was firmly entrenched in this movie but I appreciated the 2004 movie because it gave me a different viewpoint on the myth of Arthur that spurred me to doing a little research on the validity of its claims. Turns out they got a number of things wrong, but I still liked the idea and the effort.
Let's face it, we KNOW he did not exist in the 12th - 16th centuries, but what we don't know is if he ever did, and if he did, when? At least, that's my take on the subject.