If I'm to understand this right, this question seems to be about generating code in a target language, where the language itself someone's closely-guarded intellectual property.
For instance, the language is not even publicly documented. Let's go with the assumption that one needs to sign a non-disclosure agreement in order to know anything about this language, including possibly even its very existence.
If you distribute an open-source compiler whose output is in some secret language that cannot be discussed with others and requires a non-disclosure agreement to learn about, then your work probably violates the non-disclosure agreement by revealing that language, regardless of what copyright and licensing notices are in the output.
The open source compiler you're redistributing is imbued with knowledge about the target language. It contains logic which can output pieces of that language to make a coherent utterance in that language. That logic is based on knowledge can only be obtained under NDA. So by redistributing the compiler, you're leaking the knowledge, breaking the NDA. The NDA forbids you from sharing knowledge with parties that are not under the same NDA, regardless of how you share that knowledge.
The licensing of your compiler, and the copyright notices you put on the target output written in the proprietary language, are not directly relevant. If your compiler leaks secret information obtained under NDA, it doesn't matter whether your compiler is copylefted open source, or whether your compiler is closed-source
.exe file for Windows given to users under a shrink-wrap EULA.
This is an area where, if circumstances are compelling you to proceed with redistributing such a compiler, you need lawyers. Likely, though, they will tell you the same thing.