I assume this was a grift — they "found" some bodies and declared them Arthur and Guinevere, and suddenly the site became a pilgrimage destination and lots of money started flowing in from the pilgrims.
Says right there: they dug to find them. Actually finding them might have better chances elsewhere, on account of the bodies being elsewhere, but the light was better here and they didn't have to walk that far.
Bingo. There is no real historical proof that Arthur existed (that I know of), all historical mentions of him are likely themselves taken from already poetic sources.