The precursors were not the Flood. The precursors created the Forerunners as slaves. The Forerunners rebelled and assumed the mantle. The Flood are a bio weapon developed by the precursors. The Humans accidentally stumbled across it and it slowly evolved and infected their "dogs". It eventually became known as the shaping sickness by the humans and the flood by the Forerunners. The gravemind Mendicant spoke to was a precursor mixed with a variety of other species.