It says so right in the OP

> The original work by Bjorn Staal used localStorage, but I found sockets more fun, because if tweaked a bit, this can be shared with friends.

I might be wrong, but I think the point is to simulate a server-client architecture (to show that it works even for an actual web app).

those were my thoughts too

You can use Broadcast Channels as well as Service Workers, even WebRTC would use a loopback interface!