Well by that definition Docker, Kubernetes, etcd and so on are systems software. But people here somehow explicitly make it to mean Computer Operating Systems.
Sure, and Go's most prolific users are in infrastructure software for distributed systems. You seem to agree with the quoted statement.
Systems does mean that. There are 2 broad categories of software you can write. One is software that provides a service to the user directly. That is an application. The other kind is software that provides a service to applications. That's systems software. Do you think there's something wrong with this notion? It's pretty well accepted over the decades:
http://en.wikipedia.org/wiki/System_programming