While I think their claim is not credible (and their argument has an obvious logical flaw), their claim is that the plain polished stone cup underneath the gold and jewel setting is the original, and that the setting is a much later decoration.
That seems possible. We might never know what happened to the Grail. Maybe the Lord zapped it back to heaven to be on His memento shelf, for all we know. The meaning of Christ was symbolized by the grail.
were it indeed The Cup...who in their right mind would have decorated it?