Only if you see source code as the only valuable thing, which it isn't. The knowledge of the team, industry connections, experience etc etc are a big part of what make it so you can effectively use the source code.

We're making an industrial sorting machine. Our management is feared to death to lose the source code. But realistically, who's going to put in the time to fully understand a codebase we can barely grasp ourselves? Then get rid of all custom sensor mappings, paths and other stuff specific for us. And then develop on it further, assuming they even believe we have the "right" way of doing things?

Right, no one. 90% of companies could open source their stuff and, apart from legal nonsense, nothing practical will happen, no one will read the code.