That was their excuse, but was it true? Did they know that Ancient Humans and San Shyu'um would feed their dust to their pets, who would mutate and become the Flood?
The precursors were never mentioned in the games. They are part of the EU (Expanded Universe), so in the books. And we kinda already had them in the games. After the forerunner precursor war, the precursors tried to save their lives but turned themselves into thze flood.Actually the Precursors turned themselves into the Flood as a long-con revenge against the Forerunners.
Actually, it sounds quite believable now.
They could've intended it to be eaten, and purposely had it mutate.
The dust form was originally intended as a mechanism for long-term (very
long term) stasis, to hide from and escape the Forerunners who were slaughtering the Precursors.
The dust-stasis failed somehow, and the Flood (which was obviously bent on revenge) was the result. So, essentially the failed Precursor stasis dust became the first form of Flood "spores", which leads on to the rest of the Flood's history. Revenge wasn't a Precursor plan, as far as we know, just certainly a Flood one.
At least that's the information we have to go on. It's possible the Precursors intended to come back and get revenge on the Forerunners after their dust state, but it's also possible they would have simply gone off to work on another galaxy. We don't know what their end-game was at the time.