I think that's the exact point the parent was making. They don't want it open-sourced, because they already can access it, and open-sourcing would only mean that good guys will have access to it too.

I don't think this is a realistic depiction of the threat model of people who would hack commercial jets. Sure, state sponsored APTs probably could access it if they wanted, but none of them are in the business of crashing passenger planes. Maybe tracking them or grounding them or stuff like that, but they already have those capabilities through economic or bureaucratic or military power. The "bad actors" of concern here are the same types that have brought down passenger planes before - small, independent groups or lone wolves. And they are the ones who would benefit from the code being open sourced.