Judges, as far as I know, do not generally take kindly to such arguments.
Any other interpretation would mean trade secrets have essentially infinite protection/NDAs apply to all of humanity.
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.
That's what's referred to as a "clean room implementation" further up the thread.
However, this specific thread is about the scenario where an employee has inside knowledge and is passing that knowledge on to the implementer.
Legally speaking, a clean room implementation has much better defenses from claims of copyright violation.
This sort of thing has always been a concern for projects that re-implement some existing piece of software, e.g. there were concerns early on during development of the GNU project about people contributing after having been exposed to Unix source code. Similarly, it is an ongoing concern for the Wine project[1], as well as ReactOS. The later even had a code freeze and internal audit in 2006, after allegations came forward that code had been derived from disassembled Windows binaries[2].
[1] https://gitlab.winehq.org/wine/wine/-/wikis/Clean-Room-Guide...