The prisoner is indeed a precursor. The precursors created, and or adapted the flood for their own needs
Precursors created Forunners. They were the gardeners of the galactic gene pool. You see, a long time ago the precursors became obsessed with elevating themselves to the highest levels of technology and evolution.
This actually turned ot to be simpler if you got others to do the work for you.
create forunners > they nurture the galactic gene pool > use the ability of their flood powers to absorb the advantages of all that diversity and cognitive ability form a galaxy full of sentient beings.
you liek?