This is why the Forerunner Saga is told through ancient artefacts that ONI has discovered. 343 Guilty Spark states in Halo CEA's second Terminal that all concrete records of Forerunner history have been lost, so we're relying on second-hand information now. Cryptum is told from an artefact known as the Bornstellar Relation, Primordium is told through Chakas' memories when he's recovered by the Rubicon, and Silentium is a series of 'strings' harvested from the corpse of Catalog.