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.
Arthur probably didn't exist. Guinevere certainly didn't. Even if they did, there's no reason to assume any particular location would be the location of their tombs.
Plus, according to the legend, Arthur's supposed to be alive and in the mystical island of Avalon, waiting until he is needed to return and become king again. These guys needed to look up their Geoffrey of Monmouth.
Basically, it would be like digging to find the tomb of Uncle Sam and his wife.
Or they had tombs made for them and they remained empty because Arthur is alive on the mystical island of Avalon
Yaknow, pharaoh pyramid rules - they built those suckers when the dude was still alive. But one dude just didn't have to use his "pyramid" cause he floating on an island.