You've confused ethics and morals. The ethical action (which is about professional standards rather than your conscience) is usually to follow the legal agreement you've signed (barring something that supersedes the NDA like being legally required to report something to regulators).
So no, it isn't unethical for the author to abide by his NDA, arguably the exact opposite is true, though exposing these shenanigans at a personal risk could be argued to be the better moral decision.
So no, it isn't unethical for the author to abide by his NDA, arguably the exact opposite is true, though exposing these shenanigans at a personal risk could be argued to be the better moral decision.