To me, there is a confusion of what "copying" and "using" means.
You can copy the idea and not use the source code. This has been ruled ok many times already and would be quite dangerous if that was not the case.
But this is not what this is.
To generate the new program, another program, the AI, must have an input which then becomes part of the program itself.
It does not really matter much if the generation does not contain the source code itself or a similar reimplementation. One could rewrite a full version of the Lord of the Rings changing all the words but having the same elements, it would still be plagiarism. No reason to think this is not the case here. It is evident that the source code was the base, hence, this is a derived work.
You can copy the idea and not use the source code. This has been ruled ok many times already and would be quite dangerous if that was not the case.
But this is not what this is. To generate the new program, another program, the AI, must have an input which then becomes part of the program itself. It does not really matter much if the generation does not contain the source code itself or a similar reimplementation. One could rewrite a full version of the Lord of the Rings changing all the words but having the same elements, it would still be plagiarism. No reason to think this is not the case here. It is evident that the source code was the base, hence, this is a derived work.