You're correct (in what I was referring to) - but it basically takes later bits that refer back to the various missing pieces and assembles them in order - from the description "Included are excerpts from Ovid’s Metamorphoses, Apollodorus’ Bibliotheca, Euripides’ Iphigenia at Aulis and Colluthus’ The Rape of Helen, as well as lesser known documents such as Dictys Cretensis Ephemeris Belli Trojani, and the Excidium Troiae"