From Complex systems are composed of many different interacting components:

A Complex system is made up of many different components. These components not only interact with and depend on each other, but also their environment.

From What is a socio-technical system:

A socio-technical system is a system that is not just made up of technological components, but also the humans using those tools, the environment they are in as well as the organizational and social structures around them.

From these observations, it follows that socio-technical systems are by their nature complex systems.