I hear what you’re saying, but we don’t have a word for that different thing. OS means the kernel. But OS also means the entire distribution - all of macOS, all of iOS, all of windows, Ubuntu, Android, etc.
Until we have a standard word for the ui “shell” that comes out of the box with the OS distribution, people - including me - are gonna call (and think of it) as the OS. Or part of the os. Or something like that.