I think what's being talked about here is the back end implementation for ActionCable. By default it uses ruby threads to push over open web sockets. There's at least one production quality drop in implementation (https://anycable.io/) that address the default scalability issues you'll have with ActionCable. The async support would seem to allow one to go much further with default rails before needing to move to something more performant.
I'm guessing the Rails implementation for the webs sockets part is thread-based and if that's the case fibers can make a big difference. But I'm also a bit out of my depth here.
