What is Systems Thinking?
Googling "system thinking" returned a lot of resources that are about "Systems thinking" rather than "System thinking". This source indicates that "Systems thinking" is one of the three ways that could be applied in DevOps.

After reading several references "Systems thinking" is about interactions rather than silos. If one translates this to DevOps then "Systems thinking" is aligned with the theory and also important for DevOps as this is the interaction between QA, Dev and Ops.

In order to apply "Systems thinking" one should be able to communicate with people from Dev, Ops and QA, listen to them, understand their problems and find ways to let them work together instead of getting a "through it over the wall" mentality and unconnected departments in the company.

An element that really requires "Systems thinking" is Continuous Delivery (CD). Multiple departments are involved, while CI is mostly used by Dev, QA and Ops will be definitely need to be involved in this process as one does not want to release a newer version if integration or manual tests failed and Ops want to be informed if there are changes to the system as they monitor systems and try to prevent and solve issues on the platform.

In summary, "Systems thinking" is required for DevOps engineers. If one only sees Dev, Ops and QA as individual parts, but no interdependency then this is not aligned with the DevOps theory, i.e. intersection between Dev, Ops and QA.

References

https://medium.com/disruptive-design/tools-for-systems-thinkers-the-6-fundamental-concepts-of-systems-thinking-379cdac3dc6a

http://www.thwink.org/sustain/glossary/SystemsThinking.htm


  a discipline for seeing the 'structures' that underlie complex
  situations, and for discerning high from low leverage points.


https://thesystemsthinker.com/connecting-systems-thinking-and-action/


  The iceberg is one of the most frequently used models to explain
  systems thinking (see “The Iceberg”). Thanks to movies like “Titanic,”
  many people recognize that most of an iceberg sits hidden beneath the
  water; that is, out of sight.


https://www.accenture.com/us-en/blogs/blogs-devops-evan-erwee-system-thinking


  System Thinking allows people to make their understanding of social
  systems explicit and improve them in the same way that people can use
  engineering principles to improve their understanding of mechanical
  systems.
  
  ...
  
  We can therefore state that System Thinking for DevOps is required to
  understand Software Engineering as a social construct using
  engineering principles.
  
  From a System Thinking approach, DevOps thinking is fundamentally
  different from the traditional form of analysis. Traditional analysis
  focus on separating the individual software engineering pieces, e.g.,
  architecture, coding, testing, tooling etc. In contrast, the DevOps
  approach focuses on how all aspects of the software life cycle
  interact with other constituents of the system. Circling back to the
  definition of DevOps we can easily identify the ‘behavior traits’
  observed in the DevOps movement, namely: adaptable/flexible (improve
  relationship), cooperative (better communication), diplomatic
  (collaboration), etc.


http://www.systemicleadershipinstitute.org/systemic-leadership/theories/basic-principles-of-systems-thinking-as-applied-to-management-and-leadership-2/


  Systems thinking is a management discipline that concerns an
  understanding of a system by examining the linkages and interactions
  between the components that comprise the entirety of that defined
  system.


https://thesystemsthinker.com/introduction-to-systems-thinking/


  What exactly is systems thinking? In simplest terms, systems thinking
  is a way of seeing and talking about reality that helps us better
  understand and work with systems to influence the quality of our
  lives. In this sense, systems thinking can be seen as a perspective.
  It also involves a unique vocabulary for describing systemic behavior,
  and so can be thought of as a language as well. And, because it offers
  a range of techniques and devices for visually capturing and
  communicating about systems, it is a set of tools.


https://www.burgehugheswalsh.co.uk/Systems-Thinking/Approach.aspx


  Fundamentally, Systems Thinking is straightforward. We treat the
  situation of interest as a system and examine it using system
  properties and characteristics like purpose, context and structure to
  discover more.


http://study.com/academy/lesson/systems-thinking-in-management-definition-theory-model.html


  System thinking is a method of critical thinking by which you analyze
  the relationships between the system's parts in order to understand a
  situation for better decision-making. In simpler terms, you look at a
  lot of the trees, other plants and critters living around the trees,
  the weather, and how all these parts fit together in order to figure
  out the forest.