Dude there are literally characters in the novels who are Precursors. The precursors built very real very dangerous weapons and structures that Forerunners actively revered and studied. The entire reason the Flood exists is because of them; they [i]are[/i] Precursors in there newest form. The "myth" comes from forerunners worshipping them. They're a myth in the same way the forerunners are mythologized by the covenant.