Earlier quoted context omitted.
I’m just not sure how you guarantee no changes to the API without knowing the size or scope of changes that may come to the language it’s built on.
As an example, Java does just fine at guaranteeing its own semantics completely independently of what happens to C++.
Decoupling is of course possible, but the difference here is that the lang powering the runtime is not guaranteed to remain the same language it currently is, since it's in active initial development. That introduces a unique axis of risk to runtime stability, it's dishonest to pretend otherwise.