You'd have to somehow substantiate the claim that they could defeat the Flood. The flood didn't lose, they retreated on purpose to make the humans think they found a cure. This lead the Forerunners to think they found a cure, this preserving humanity. The Flood knew the Forerunners were going to go to war with mankind and wipe them out eventually, so they convinced both sides that humanity held a valuable secret by pretending to back down against a cure that never actually worked. This saved humanity and allowed the Flood to enact it's punishment of the Forerunners for defying the Precursors, therefore allowing humanity the freedom to ascend. The entire story with the Flood has them saving the human race, as we were the Precursors chosen species. They even ended the Covenant War for us, by initiating the events that would lead to the Ring be destroyed and the Covenant fracturing. These are the kinds of master minds you are dealing with