Systems thinking says nothing about the content of thinking, only about its form — the use of systems thinking concepts in thinking. Systems thinking controls attention, and the objects of attention themselves can be from completely different domains. Systems thinking in this respect is transdisciplinary: it does not touch the concepts of applied disciplines. If cooking is an applied discipline, soup can be a system. If the applied discipline is quantum computing, the system may be a quantum computer. But systems thinking will drive attention to the objects of other disciplines: pointing out the important things you should not miss during thinking.
The transdisciplines developed on the basis of systems thinking (systems engineering, systems management, etc.) do everything so that one does not have to think much but can simply apply in the project already known technical, managerial, creative solutions. There is no need to invent the idea that requirements have to be documented. This has already been discussed many times, you just need to leverage this solution: if you have an engineering project, the requirements need to be documented. If you don’t, it’s going to be wrong; you know that in advance. You have to save your thinking and document the requirements right away, not wait for the effects of undocumented requirements to show up in the project. But which exact requirements to document? This requires thinking; systems thinking and systems engineering will not tell you.