In the US, a contract can’t supersede laws like those that protect whistleblowers. (I think this is part of how Harvey Weinstein was prosecuted because his NDAs were found invalid)

The author didnt disclose any illegal activities in her book. And she didnt claim whistleblower status.

I doubt such clauses can prevent you from disclosing them to relevant authorities. Disclosing them to the public is a whole other matter.

I don't think such clauses have ever been held to prevent people from testifying in criminal trials. Signing book deals on the other hand...

If it's true that she signed a severance deal, e.g. signed this when she was leaving and therefore already knew she was agreeing to protect a bunch of snakes.. Well she fucked up. At the point when she signed that agreement she was already informed and aware of what kind of people she was agreeing to not disparage.

At the end the book is published.

I do. Illegal activities can be dealt with by the legal system without needing to be publicized first.