Reason why ONI kept the Great Schism classified

[Accessing]

In Halo 3: ODST the great schism is shown as the closing fight for New Mombasa is shown. As Alpha-Nine exits the city you can find dialogue and clues to Brutes killing Elites throughout the city in what is a little early on in the timeline. ONI and Dare notes this as ONI classified material and should not be told to the mainline UNSC. This recently came to mind while playing which leads to a few similar explanations or all. 

The Brutes were taking their orders directly from the Prophet of Truth. And the Prophet was preparing for the Brutes to take over for a long time behind the scenes. He was even preparing a large Fleet specifically for that reason alone. But ONI's actions here are centered around how trustworthy the Sanghelli were, or if the infighting was universal across Covenant forces. They probably envisioned Marines potentially lulled into a false sense of security by the Elites. 

In addition to this, we know from the intelligence process that they had to verify and validate this information as true before disseminating to the rest of the UNSC. Just food for thought, but do you think there could be other explanations?