Since this article is specifically about Javascript: I think the implementation of async/await in the language is heavily constrained by a desire for backwards compatibility.

After all, async/await is ultimately promises all the way down, so you can happily write code and let a compiler turn it into ES5 that runs anywhere.

If instead JS allowed you to shunt arbitrary function calls off into their own threads, then you would need bigger changes to the engines to support that, and you couldn't back-port that to vanilla javascript.