I have not seen the book and so can only go off the reporter's summary which may or may not be accurate.
Apparently the two individuals claim to have discovered something in a neglected Syriac document by Zacharias the rhetor, who was bishop of Mitylene.
Scholars scrutinized the document and discarded it as insignificant.
The Sunday Times quoted Wilson describing it as an “ancient Syriac manuscript lurking in the British Museum…. Scholars have known about it for almost 200 years, but have not known what to make of it.”
There are two ways of looking at this. On the one hand a neglected Syriac document is something of a tautology. Syriac studies are largely neglected. (For those who do not know Syriac is a dialect of Aramaic that is used by Christians.) On the other hand, for Syriac scholars, Zacharias' Ecclesiastical History is comparatively well-known. There are at least three editions since 1800.
Reading the treatments of Wright, Brooks, and Baumstark it hardly seems as though scholars did not know what to make of this document.
They [Wilson and Jacobovici] claim the meaning of the text had been shrouded in code and “embedded meaning.” It speaks of a figure named Joseph, who apparently bore striking similarities to Jesus. He was depicted as “savior-figure,” the book said. “Joseph, like Jesus, was assumed dead and turned up alive; he too had humble beginnings and ended up a king of sorts.” So they contend Joseph was really Jesus in the text.The sixth chapter of the first book of Zacharias is a translation of the pseudepigraphic work of Joseph and Asenath from Greek into Syriac. The copy or the translation is generally thought to be slightly garbled or corrupt. The general consensus is that the text refers to Joseph, not Jesus, and that this is a known work. Based on what I have read of the Syriac text, I would agree that it is about Joseph and not Jesus.
We will have to wait for the book, but the argument looks like typology run amok. In the meantime, I suspect that this is a bad argument sensationalized.