Absolutely, that's indeed the reason behind GNU/Linux as opposed to GNU Linux.

Linux is not a GNU package and hence not GNU Linux, however when it includes the GNU userland, it runs many GNU packages, hence GNU(userland)/Linux(kernel).

RMS is not taking credit for Linux and does not want anyone to call the kernel GNU/Linux, but rather the whole system IF it is using GNU userland.

But my original point is that the overwhelming majority of Linux systems include a lot of non-GNU software to achieve basic functionality. Why not include all that when referring to the operating system?

Because GNU/Linux is the very core you need to have even a remotely functional system, i.e. you need both GNU and Linux to even have an OS of some sort, the other parts are used to make the OS even more useful, but they're not the parts without which your system will fail to boot into a reasonable state to even get these other pieces working.

In Windows there is lots of software not written by Microsoft, then there's the NT kernel, (which by itself does not make an OS) and the userland (which together with the kernel makes a basic OS that you can install all the other nice pieces onto to have a great experience, but the core is NT + userland)

You still need GCC (Clang still can't compile the kernel).

That's usually not considered part of the distribution itself, though, and thus not really a contributor to the discussion of "GNU/Linux" v. "Linux". That'd be like calling pre-Clang versions of Mac OS X "GNU/OSX" just because Apple used GCC to build their operating system, or calling OpenBSD "GNU/OpenBSD" just because OpenBSD still uses GCC.