You are conflating criminal libel with whether an NDA — a contract between two private parties — is enforceable.

Courts can refuse to enforce private contracts if the objects of the contracts are against public policy.

Let’s suppose the NDA is enforceable. The NDA still can’t work a prior restraint on publication in all probability. Prior restraint is disfavored in US law…disfavored big time. I can imagine a case where the NDA is with the government and publication would implicate national security. Even in that case, prior restraint would not be a slam dunk. Remember the government suing Daniel Ellsberg (and major newspapers) over the Pentagon Papers?The remedy for the violation is money damages and if there is no libel and/or truth is a defense, it’s hard to picture what money would be lost.

US courts have all merged law and equity, so the court could put on its equity hat and take the position that it’s not fair for the author to profit from violating a NDA and order the publisher to pay the royalties into the registry of the court to be fought over later.

The matter of bringing truth into it is an often overlooked part of New York Times v. Sullivan, the case that painted targets on the backs of public figures by requiring proof of “malice” to show a libel. At common law, all you had to prove to make a libel or slander case was defamation. If the speech was defamatory, the author had to respond in damages without regard to whether the defamatory statement was true. Also, Merrie Olde England did not have the hangups we in the colonies have over prior restraint, so it was possible to enjoin publication if you moved quickly enough. It’s “possible” on this side of the pond but not bloody likely.