Unfortunately such a claim is false. The event loop and callback oriented asynchrony model is baked into the ES spec itself; future ES features rely on it explicitly.
You are probably confusing ES spec with DOM spec, if not, at least I cannot find anything in ES spec even remotely related to how anything built on top would have to be asynchronous.
The concept of an event loop and event loop 'turns' is backed into ES promises the last time I read the draft spec and kept up with the spec discussions. It's not something you can polyfill in older versions of JS, it's a platform feature designed around how browsers work.