This case is clearly not over the line but it seems to me that the minimum can't be a direct use of code. If someone did enough documentation of the architectur

This case is clearly not over the line but it seems to me that the minimum can't be a direct use of code. If someone did enough documentation of the architecture and algorithms used in the Linux kernel and handed that off to someone else who reimplemented I assume it would be hard to argue it wasn't a derived work. When LWN does a description of how the VM or scheduler works they are effectively describing the code. But maybe that's the same as a research paper on algorithms where reimplementation is clearly possible. Maybe for it to be a derived work you'd have to describe the code so exactly you'd be effectively translating it in whole to another form.