"The plugin does nothing by itself, so you can argue that it is irrelevant as a piece of work and that only the whole thing matters."
You can, and get laughed out of court, unfortunately. Legal arguments actually have to have a legal basis, not just be an exercise in idle speculation.
"Indeed it would look silly if #include "foo.h" was subject to different laws than "import foo", whatever these laws are"
It depends on whether you are making a source or binary distribution. If you are making a source distribution, you don't have to distribute foo in either case. If you are making a binary distribution, it makes a _big_ difference whether protected elements of the include files are substantially included in the resulting binaries, or just referred to during the compilation process. This is not some new thing. The Supreme Court set the basic precedent in this matter more than 110 years ago.