The NDA doesn't apply to people who haven't signed it, but copyright law does. If you know the material you're receiving is under copyright (eg proprietary source code), and you publish work based on that copyright material, the fact that it's now widely available is not an effective defense against claims of copyright violation.
Person B in this scenario hasn't violated the NDA, but they could be sued for copyright infringement.
Which I think you know, so I don't understand your argument.
You said in another comment "However, this specific thread is about the scenario where an employee has inside knowledge and is passing that knowledge on to the implementer."
It being proprietary and/or secret information doesn't affect the copyright of the clean-room output. If there's trade secret implications those are worth discussing, but as a separate issue.