Craig Evans, one of the world's foremost Jesus scholars, was asked about this.
Baigent describes how he went into a walk-in safe of an antiquities collector and saw the papyri under glass. He couldn't take a picture of them, of course. He has admitted that he doesn't read Aramaic and said the other guy doesn't either--so how does he know what they say? He's assured that two well-known archaeologists, Yigael Yadin and Nahman Avigad, have confirmed it. Oh, but did I mention that Yadin and Avigad are dead?Yep. But the fact that it is nonsense doesn't stop gullible people from believing it.
So we have an author with dubious credibility in the first place; an antiquities dealer who can't be identified; documents that Baigent can't read or produce and for which we have no translation or verification; and two archaeologists who are dead. This is just the dumbest thing (Craig Evans as quoted in The Case for the Real Jesus by Lee Strobel. Grand Rapids : Zondervan, 2007. 52-53).