It may run on multiple threads but I don't think it exposes threading to the runtime. Ecmascript is certainly single threaded and async by design, so v8 would be straying far from the spec.
for opening up a new separate thread, that you can only communicate through message passing. It's not part of the core language but the browser library hence why you can't use them in Node.
They're also more directly comparable to processes than threads, as they do not share memory (though there are a couple of proposals to allow them to).
I'd love to be proven wrong.