Maybe that is one reason for looking else where. Also, King Arthur is widly believed to have died at this same 540AD date.
Can you reconcile this statement with the one below I found in the link you provided to Madoc In America?
"We now know who led the 700-ship expedition to America; the records say it was Arthur II ap Meurig, along with his brother Madoc Morfran and brother-in-law Ammwn Ddu."
FGS
Sure, King Arthue I and King Arthur II.
Any history is always told in the senses of the time in which the historian writes it. It tells as much about the time in which it was written as the time written about, if the reader is deep enough to read for it.
And too, the reader of any history so written reads and interprets through his lenses and understandings which are of the the reader’s own time.