How would a team verify this for any current model? They would have to observe and control all training data. In practice, any currently available model that is good enough to perform this task likely fails the clean room criteria due to having a copy of the source code of the project it wants to rewrite. At that point it's basically an expensive lossy copy paste.

You can always verify the output. Unless the problem being solved really is exceedingly specific and non-trivial, it's at least unlikely that the AI will rip off recognizable expression from the original work. The work may be part of the training but so are many millions of completely unrelated works, so any "family resemblance" would have to be there for very specific reasons about what's being implemented.