Well, NT runs on top of a HAL so could be considered not running on bare metal. And some systems have supervisory code running in a coprocessor. At the other end, a modern OS, like the mainframe OSes of yore, did IO by talking to coprocessors that did the heavy lifting. In the old days we called them "channel controllers" but now we just say that the USB chipset handles the details with the OS just orchestrating.
But I agree with you, as I hope my comment was clear: while it's an interesting metaphor, K8s is a long way away from being usefully thought of as an OS. And if it ever does reach a point where such a term would be useful it will be unrecognizable when compared to where it is today.
My guess would be they mean running a specialized unikernel.
Keep in mind, any such kernel would have very simple requirements. k8s typically runs on a hypervisor not bare metal, taking care of security, and bare metal compatibility. k8s manages the sharing of resources. The file system typically needs to be ephemeral rather than persisted. IPC all happens through explicit APIs.
You must have miss-read my comment or I did a poor job writing, sorry.
I was saying, to become an OS, “there is very little OS functionality that k8s would need implement” most of the functionality is already handled by other parts of the stack, e.g. hypervisor, networked storage like S3, and user built APIs.
A bit of a stretch; k8s has a long way to go before it fulfils the requirements of an OS. But it's an interesting way to think of it.