I guess you could say Da Vinci Code is a gonzo novel. It contains many nuggets of truths, facts, and history, and placed in an otherwise realistic world, but it ultimately is a fictional world and facts are blended with fiction to tell a story.
The holy grail is not one of those bits of reality. And, seriously, Jesus would probably facepalm his palm through his face over such a silly idea of a cup he drank from being a sacred relic, equally absurd to the idea of him having a divine bloodline, which is made utterly redundant by these descendants being unaware of their lineage.