Whatever you are looking at is just talking about the compiler. Something you can do in English is, when referring to something in particular, you can say "the X proper" or "the X itself." Sorry, I speak English as a native language so my ability to explain nuances like this are about nil. :P
The GCC toolchain is a good example. It includes, at its core: ■ the preprocessor ■ the C++ front end lexer and parser ■ the backend compiler ■ the assembler ■ the linker
and additional tools like automake, make, autoconf, libtool, etc.
When we talk about the GCC, we often mean the the whole bundle -- the things that happen when you type
at the command line.
When talking about the compiler proper -- you are excluding all those parts of the toolchain that are not properly called a 'compiler': make, preprocessor, lexer, assembler, linker, etc.
Every person is an expert in his own language -- he has studied it his whole life. Being able to explain why a thing is is a bit different than explaining what a specific idiom means -- which you did nicely.
I have not seen any evidence that cppuser is not a native English speaker -- he has demonstrated a pretty firm command of it so far. An idiom like "x proper" is not as common as it used to be outside of computer science circles... So it isn't too much of a stretch to say he had just never heard it before.