Based on the article and the fact that they implied that sticky sessions are necessary, it doesn't seem that the abstraction is airtight.

If it was properly stateful, then sticky sessions would not be required. With WebSockets, sticky sessions are not required when load balancing.

It's not sticky sessions in the traditional sense. It's a single connection. Http/1.1 requests are allowed to be stateful within a single connection as well. The difference is there is built support for multiplexed requests within that single connection.