From personal experience as far back as the 80's (and from my understanding going back before that as well), OS has never meant kernel.

An Operating System is the collection of software that allows you to operate a computer, so that means kernel, program loader, simple text editor, simple disk management, etc.

As computer users became more savvy, and hardware became more powerful, more and more functionality was included in the OS (graphical interfaces, utility apps, etc.).

I don't think many people would have trouble calling Android an operating system, and that's just the Linux kernel with utility apps, loader, and app libraries, yet very different from something like Redhat.

I don't think it's a stretch in the least to call Horizon an OS.