Flood? I mean look how quickly the Flood evolved from where they were in Halo: CE and Halo 2, then how it evolved by Halo 3. Now do you think possibly the Precursors have been evolving the Flood over the missions of years?
the entire point and theme of the flood is how they constantly adapt.
In short. Kill, Consume, Adapt. “It is unfortunate that their overriding objective is to butcher and consume every species in the galaxy” (Bestiarum. Can’t remember the exact line…My memory’s slightly fuzzy on that)
With the primordials line of “The precursors are the flood, there is no difference”, i’ve generally taken it as the precursors are simply a highly evolved version of the flood.
My theory about that is that the Precursors used dead Precursors to create the Flood, hence the Primordial saying there is no difference.
Or some precursors gave themselves to the flood to enforce their view of the mantle. The again they may not now be a difference because the flood infected all of them.