Canon, schmanon. This comes from the games, doodz.
The 483 (firstname.lastname@example.org) writes:
A line in the cutscene [Floodgate - ed] at the end of the first invasion of the flood on earth suggests that the elites know more about the flood than they let on. Specifically, right after they watch the message Cortana sent, the elite shipmaster, Half Jaw, say to lord Hood,
"You heard your construct, the flood, a Gravemind, is on it's way
and then later the arbiter remarks
If the construct is correct, then you've already lost."
The way they refer to it as "A" Gravemind, instead of "The" as cortana and the Chief have, seems to say that the elites, or atleast higher ranking Covenant officers have dealt with, or seen materials regarding Graveminds, and have a much deeper understanding of the flood than they have mentioned. This would help us to understand why Half Jaw himself, who is probably in command of the entire covenant portion of their navy, chose to chase the single flood infested ship to earth, and leave the (failed) containment of High Charity to the lower ship masters. Besides the chance to get revenge on "The Bastard Truth!" of course, Knowing earths potential to give the flood sufficient proteins to form another Gravemind.
Following this line of thinking, it's safe to assume that the Covenant (Prophets Hierarchs) knew full well what they were unleashing when they landed on Halo 04. Or, they could have learned when the captured Guilty Spark from the heretic leader from Threshold.
More evidence that the Sangheili rock way more than us? Or just an odd speech pattern?