They are not, BTW, legends ~ just that you don't know what the Breton word for Arthur is, nor it's historical use ~ try Boadicea, which is both the name of a British revolutionary in Roman times, and a sentence. "Bo" means the same as "Mc", or Abu in Semetic languages ~ that is, paterfamilias. "a" or "ad" is "Arthur" or "military leader". "dic" (with a hard c) is a fairly universal term meaning "king", "lord", "ruler", etc, but here it's feminine in form suggesting we are talking about a great queen.
So, there's your "Arthur". How it got preserved down to the 6th century (538/540) is a real good question.
BTW, both of the aforementioned documents were mostly written BEFORE 540, and that Excidio contains few real dates!
Now you are really stretching.
De Excidio Britanniae and The Confessio of Saint Patrick are the ONLY two contemporary sources concerning the 5th/6th century ~ outside of the Arthurian materials in the Welsh Annals.
Only two sources of what? Arthur or the general history? Because there are many documents from the 6th c., so where are the contemporary accounts of your mega-disaster? (I think we can safely dismiss 5th c. sources of what happened in 540 AD, don't you?)
I mentioned De Excidio Britanniae, because it was likely written in the 540s, and so exactly when you claim the event occurred. If that isn't exact enough for you, how about Historia Francorum, by Gregory of Tours, written c. 591 AD? Surely such a disaster happening only 50 years before would be mentioned in a definitive History of the Franks.