Good introduction to nginx. However, the guide states: "Keep in mind that the maximum number of clients is also limited by the number of socket connections available on your sytem (~64k)".

This is incorrect. The system can open ~64k connections per [src ip, dst ip] pair. In the case of a webserver listening on just 1 port, it means you can open 64k connections per remote IP, which is why some people can write about how they handle a million connections on a single server.