The first i’d heard of this theory was several years ago, when the, probably apocryphal, story of a Welsh pastor being captured by injuns in the 1700s. He recited a prayer in Welsh, and his captures understood him!
Cassiodorus reported strange atmospheric phenomena from 534-6, which involved weak sun, much summer cold, crop failures, strange color of the sky, etc. There are also Chinese records of that period reporting famine, etc. Perhaps Arthur II was killed in 540 by the plague that swept Europe about that time caused by the previous years of famine and subsequent population weakness. Then about 20 years later 562 the next generation, probably with the same or similar names sailed for the new world. Gloria Farley’s book has many interesting examples of Celtic writing and religious figures up various river systems.
It just occurred to me, doesn’t ap mean son of or like junior? Or am I remembering this wrong? If true, then it strengthen’s my previous comment.