Tiro's writings did not fail to survive because of the sack of Rome, they failed to survive for the same reason thousands of texts failed: they stopped being copied. The literary output of the Roman world was immense, unparalleled by anything until the modern era, but copying texts was a laborious and expensive process. Texts that were not as popular were not copied as much, with the result being that only a short period might be enough to endanger a text's continued existence. Catullus had a very brief period of relative unpopularity under the Antonines, and already by late antiquity authors are lamenting his near-total loss. Sappho, who was popular among poets and the educated throughout antiquity, is lost because her works were never used in the Greek school curriculum. Especially in the Middle Ages, from which almost all of our copies come to us, it was not always profitable or even possible for scribes (especially monks) to devote resources to maintaining these texts that no one particularly wanted. Besides, the sack of Rome would not have had any significant effect on a text of any importance. It's not as though all copies of Tiro's texts were sitting in the city itself.

We cannot know what was in Tiro's history. We have some references to Tiro's writings--Plutarch cites Tiro as his source when he says that Cicero married Publilia to erase his debts, for example--but on the whole we do not have significant fragments of any of the various literary works he produced. Harris uses the various ancient accounts of Cicero's (and his contemporaries') life, as well as Cicero's own speeches and letters to construct a narrative that he labels in his fiction as Tiro's work on Cicero. But this is as much a fiction as Robert Graves' conceit that I, Claudius is a translation of a newly-discovered Claudian autobiography. Harris isn't seeking to reconstruct Tiro's biography, it's just a literary device. There's some reason to believe that, for example, Tiro's biography probably concurred with a lot of the stuff we read in Cicero's letters and speeches, which Tiro edited and used as a source. But we have pretty much no idea what was in it, not very much of the work survives in quotation or reference at all.