The proposal only is for sharing memory in between Worker threads, which can't access the DOM (which also means canvas, WebGL, etc aren't able to be accessed). It doesn't do anything to help make UIs smoother: you still have to copy to transfer data from the UI thread to the Worker threads and back. If that isn't a problem, then Workers as-is will unblock your UI: you can already do this with Workers as they're implemented today.

All that this does is make algorithms running inside Workers faster. It doesn't unblock UIs, since Workers already do that if you're willing to accept copying. If copying back and forth from the main UI thread is too slow, this proposal won't change that.