To not disseminate classified information.

As long as nothing in the book is government classified information, it doesn’t matter.

And if there is classified information, this person has much bigger problems than violating an NDA.

And when I represent people all the time who write books of their time in the CIA, or time in the Air Force, they submit their books for prepublication review, and it's culled for classified information only. The courts have made it very clear over the last four decades that there is no legitimate interest in the government in prohibiting unclassified information from being disseminated.

https://www.pbs.org/newshour/show/nondisclosure-agreements-are-rare-in-government-heres-why