"Holy Blood, Holy Grail" did not claim that Jesus survived the cross. It suggested that Mary Magdeline, pregnant with Jesus' child, emmigrated to southeastern France and settled there. The rise of the Cathars in that region lend some support to that notion, but, by no means, confirm it.
How would the rise of the Cathari lend support to that crazy notion? Almost every possible negative description of Catharist doctrine was given by their critics - how could they have omitted such a glaringly heretical claim?