For one thing, it deals most likely with the functionality of the fedgov flying out of the US, during the complete shutdown of all air traffic on 9-11.... the flying out, on US planes of the Bin-Laden Saudi family members.
That kind of stuff.
Maybe methods and sources then that are still in place.
But. they should say what is sequestered in general terms, not security speak.
That is absolutely true as well. There is a great documentary out there called “9/11: Clear the Skies” about the FAA/military having to clear 4,800 planes out of the skies on 9/11. But there are hints that there are a lot of secret things involving the relocation of leadership in an emergency.