I have spent a good many hours trying to get my head around whether systems thinking and Lean are different, one and the same thing, or whether one is the application of the other. In this article Michael Ballé addresses that exact question and concludes that they are complementary and based on similar insights. He argues that ST is a philosophy while lean is strictly a practice. According to Ballé without an understanding of ST you cannot get lean right; at the same time, without the practice of lean techniques, it is difficult to make ST a concrete reality. For those amongst you with the appetite for the full article, click here. It contains some interesting stuff about the origins of lean.
In my experience, good lean practitioners are intuitive systems thinkers, even if they have no idea what on earth that means.