Usage of it, on its own, does not tie you to a specific runtime.

Using runtime-specific features and APIs ties you to a specific runtime.

Is this just an artifact of the current work in progress state of things? Is it just a matter of the interface settling and library authors providing a bit more configuration options to become runtime agnostic?

I am not a super expert in all of the exact details, but my understanding is that generally, there are a few features that don't have a common trait yet. For example, "spawn me a new task." Once those traits exist, libraries can be written against them, and it'll all be agnostic. But you only even run into that kind of problem if you need that specific API in the first place.