This is addressed at the end of the article:

  The catch

  You're relying on garbage collection, which is nondeterministic. You don't get to know when the suspended function is collected. For our use case, that's fine. We only need to know that it will be collected, and modern engines are reliable about that.

  The real footgun is reference chains. If anything holds a reference to the hanging promise or the suspended function's closure, the garbage collector can't touch it. The pattern only works when you intentionally sever all references.