There's systems like Talos where you are pretty much explicitly not allowed to touch Linux, where essentially the only thing the computer presents that you can control is Kubernetes itself. At that point, the OS is kubernetes basically; I find it much harder to entertain pretenses that it's not when you get that far along. https://github.com/siderolabs/talos

But it's really a gradient. As more and more services and systems move from being systemd services (or whatever) to in kubernetes, I see it as the position along a gradient shifting. Both are operating systems. They're different operating systems, doing different things, but both manage and provide resources for user payloads, both run apps, both host a user interface (apiserver for kubernetes), both have state (also apiserver, vs file-systems).