A common way the NLRB examines these types of things is by how a reasonable person would perceive it. Even if a lawsuit was filed to challenge the enforceability of the NDA and it was found unenforceable, the NLRB would still be able to argue a “reasonable person” would believe it was enforceable, and accordingly any restrictions on protected rights presented by the NDA would be unlawful.
That’s exactly the NLRB’s point. The NDA doesn’t necessarily have to be legally enforceable to be unlawful.