Unfortunately it seems that you're reading it correctly - the module won't do anything if the user does not have the listener active, so they need to have the page open.
If you want to deliver notifications, you're better off looking into this documentation: How to make PWAs re-engageable using Notifications and Push. You might have to play around with javascript actions to get this to work. An alternative would be looking into the free tier of Firebase cloud messaging, which can also help you get notifications to your users.