As several comments have already pointed out, Workers really are multi-threaded.
Some points which may help clarify your thinking:
- JavaScript is a language, it doesn’t define a threading model, it’s not necessarily single threaded
- Most browsers have historically been single threaded (though that is changing rapidly: IE, Chrome, Firefox), and most JavaScript implementations occur in browsers
- Web Workers are not part of JavaScript, they are a browser feature which can be accessed through JavaScript