First, here's the actual guide: https://google.github.io/styleguide/cppguide.html#Names_and_... . Of course they still have to use stuff like #include <windows.h> but it's very very limited.

They also are not removing any info -- most of it IS project headers. To me that's the actual bonkers bit :)

With GCC/Clang, headers found in paths passed with -isystem are headers that are immune to compiler arguments like -Werror because they are, by definition, out of your control. In Google's case, ALL code is already checked in the project repo, including language stdlib. So none of them are system headers "per se".