JSLinux is not magic - it too does not have raw access to TCP sockets required to terminate SSH in the browser as you think it does.
As described in its FAQ, it uses websocketproxy[1] for network access. This is a separate server application running on https://relay.widgetry.org/ . JSLinux absolutely cannot access raw TCP sockets, and ergo cannot terminate SSH connections, without it connecting to this websocket proxy. You can see this even in the Network tab of devtools.
Sure, you could spin up a websocketproxy yourself, but as I explained in a previous response to you, the value proposition of Shellvault is that you don't need to spin up any servers and host them yourself - this service does it for you.