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non-life
I found this "non-life" comment very interesting and informative. It makes more sense, but still leads to the same conclusion I came to. The flood is this "non-life" and the forerunner's "last hope" is the activation of the rings to starve out the flood. I believe the flood survived by going into a catatonic state for all the years untill the rings were found again. This may refer to the gravemind's "slumber".
On a sidenote, the second hidden message seems to refer to a forerunner civil war between those who supported the halo ring idea and those who wanted to live on and try to wipe out the flood without killing all life in the galaxy.