I believe they're referring to the essential complexity of the domain. An operating system has some "lower bound" on how complex it can be, which is rather high

I believe they're referring to the essential complexity of the domain. An operating system has some "lower bound" on how complex it can be, which is rather high. But some stuff doesn't _have_ to be hard. Like OP said, there's no inherent reason for error messages and failures to be cryptic or documentation to be sparse. The system could be more transparent or modular, easier to debug/analyze, etc. I think there's an example in GCC vs Clang here, but I'm not qualified to comment.