Seriously though - the software itself would be separated from secrets architecturally. And because it's open and anyone in the world can contribute it could be superior code than what some private government tech contractor could come up with.
For the equipment use case like jets and missiles, a separate directive component (driver) would likely be necessary anyway, not just for security/privacy but because different nations have different equipment. We use F-16s, M-16s, Autel drones, etc.
This driver/directive component would be the manufacturer's IP - like an nvidia GPU driver. Think of it like we'd all be running/contributing the same OS but because we have different hardware and security needs there are still some private components.
The idea/goal here is that our defense companies would overall benefit from it. Not just because the software is improved, but you can justify funding, build curricula around it - might benefit the defense industry to modernize their tech practices.