I would say it is an interesting fact, since it's this "good 'API boundary'" that, as you said, enables one to separate concerns, be it between different teams in an organisation or between a service provider and its users.

Yes, you don't need Kubernetes to come up with your own implicit or explicit API boundaries, and these might not be needed for smaller projects. I agree that Kubernetes is often used where it's not strictly needed.

There are things which strict abstraction, and with it, separation of concerns provide. The crucial point is that certain things are enforced.