you do NOT need to write all your programs together to make them work together.

All you say there is true -- some changes must affect several layers at once. However, a lot of changes affect glibc and the kernel at once, too -- does that mean that glibc and the kernel are the same project? (Note that for quite a long time the founder and co-maintainer of glibc was also one of the maintainers for one of the nastiest parts of the core kernel, ptrace() and signal handling, so even the people-in-common rule is true!)

I don't think they're the same project -- and neither are the various toolchain projects the same project, anymore than gnulib is the same project as coreutils merely because changes to both often happen in synchrony. They are simply projects with *some* close coupling between them -- but even that coupling is optional (you can build GCC and perhaps even glibc with a binutils that doesn't support IFUNC, and nothing goes wrong).