It looks like the server affinity is accomplished by using websockets. The http batching simply sends all the requests at once and then waits for the response.
I don't love this because it makes load balancing hard. If a bunch of chatty clients get a socket to the same server, now that server is burdened and potentially overloadable.
Further, it makes scaling in/out servers really annoying. Persistent long lived connections are beasts to deal with because now you have to handle that "what do I do if multiple requests are in flight?".
One more thing I don't really love about this, it requires a timely client. This seems like it might be trivial to DDOS as a client can simply send a stream of push events and never pull. The server would then be burdened to keep those responses around so long as the client remains connected. That seems bad.