-The Forerunners were given the mantle AND had it taken from them in the ancient past (millions of years before the time of the Didact). Then those Forerunners covered it up to the extent that literally no one knew they ever lost the Mantle.
-The Forerunners who waged war on humanity knew none of this, they didn't even learn of the existence of the Flood until the Human-Forerunner war was over. The primary reason for the war was because humanity was glassing planets with Flood infestation, including Forerunner worlds that still had spore-stage infections. The Warrior-Servants had no clue what humanity was doing other than aggressively destroying Forerunners.
-By the end of the Flood-Forerunner war, like a dozen Forerunners total know the truth about the Precursors and their relationship to humans and to the Flood, and those are the same Forerunners who survive to mark humanity as the Reclaimers of everything they built.
-The concept of digitizing humans was exclusive to the Ur-Didact, only in the final stages of the war, and only after he ran out of Prometheans to compose. He only managed to do it to one human population ever (until 2557).