Now I wonder how the test it ? Is it on a software emulator on modern equipment or do they have a Voyager replica ?

I've audited codebases in languages that I haven't programmed in. It is a matter of grasping a few basic concepts, like branch execution, branch destination, where data is stored, how it is communicated. Don Lancaster told us how to do this: https://www.tinaja.com/ebooks/enhance_vI.pdf.

Maybe only a few COBOL codebases still active can beat that? Or not?

Kids these days... Why would someone in their right mind think working on the Voyager project could damage their careers? You can work on new and fancy tools all you want to improve supporting tools, and it's still one of the coolest space missions active. Plus, it has a real end - at some point, support will be further reduced and the person will move on to another space exploration job, with the extra golden star of having been on the Voyager.

Well, the Voyagers will still be there, there will be just no way to contact them anymore once the power runs out or communication is lost (whichever happens first)...

Reading, analyzing and assembling documentation could be probably done by LLMs.

And by including old code and snippets into the training set, the LLM could be fairly proficient in writing this code probably too?

Maybe someone knows more about the use/not-use of LLMs in this context?