One thing you learn when you look into Welsh Arthuriana is that Arthur's adopted father, who is in this version Cynyr, was married twice, first to Sefin ferch Brychan, and then to Anna ferch Vortimer. What this means is that Arthur might have another claim to the throne.
In Geoffrey of Monmouth's work, there are two dynasties which hold the crown of Britain at various times. I dub these the House of Vortigern and House Ambrosia. Ambrosia is better known as House Pendragon, but Ambrosius Aurelianus's name look a lot like he was adopted from gens Aurelia, which thanks to the Edict of Caracalla is the gens most people have, into gens Ambrosia, and Pendragon is more of a nickname than a surname.
The rightful rulers are the Ambrosii, but Vortigern briefly usurps, and marries Rhowenna ferch Hengist and lets the English/Anglo-Saxons in where they take over a load of land only to be eventually overthrown by his own son Vortimer, who is praised a lot and tries to kick the saxons out. Vortigern kills his own son Vortimer, and retakes the throne. He is then kicked out of power and slain by Ambrosius Aurelianus, who is succeeded by Uther Pendragon
Anyway, through Anna, Arthur may have a claim to the throne of britain via the line of his adoptive parents. Except he has an older brother. And this is where Sefin comes in handy. If you make Cei the son of Sefin instead of of Anna, by male preference primogeniture, as well as by absolute primogeniture, Arthur becomes heir of Vortigern, since Cei isn't descended from Anna, but by adoption Arthur is. Of course, Anna doesn't inherit any of Vortigern's other lands, these beïng Powys, which went to the descendents of his secondborn Catigern, and the cantrefi of Gwrtheyrnion and Buellt went to his thirdborn son Pascent.
Anyway, maybe the reason why Arthur was given to Ector/Cynyr to raise was so that there would be somebody with a claim to the throne from both dynasties.