Then where do you draw the line?

The vast majority of people also don’t interact with the GNU tools at all, so GNU/Linux isn’t the OS either. KDE would be, or perhaps the distro itself. I’m not sure I’d call the OS GNU/Linux/Ubuntu/KDE. At that point might as well throw in firefox, for many it’s pretty much all the interaction they have with the computer.

Or what about the distros that don’t use the GNU coreutils? They are generally still called linux and still get to run apps made for linux, even with no traces of GNU.

The kernel is the OS though.