NDA applies to Confidential, Secret and Top Secret, as well as EEFI — you promise to keep secret the secrets you know.
As long as no EEFI or compromise of classified data was revealed, then no foul. This is especially true if open source information is out there to demonstrate the info was already released. . .like the leaks from the WH. And he did have a former SOF lawyer do a review. I am also sure he ran the book past Team members for their take on compromising sensitive data/info, as their LIVES are at stake.
Approval to write a book after retirement is not necessary, strictly speaking. The review is to officially get a reading on questions of classified information.
Retired officers and enlisted write and get published letters to the editor all the time. They also write books (”Nickel on the Grass” by Col Phil “Hands” Handly, for ex), and don't run it by the DOD for approval. They also appear on TV as commentators and they never obtain approval.
If classified or EEFI is compromised then they would be in trouble. . .hence the review. If on active duty, then they MUST do the review, regardless.
But other than that, no real risk at all.
Again, admittedly it's an assumption on my part, but I would think that any new specific knowledge or information that this guy could offer by virtue of his status as a SEAL and participant in the mission would still carry at least a SECRET classification.