"Six Sigma can help you improve the value-added steps and Lean can help you eliminate the non-value added activities."Yes, Lean often focuses on reducing delays and waiting time between steps in a process or between the value-adding steps in a patient's visit. There are HUGE opportunities in the so-called "white space" between process steps and reducing waiting and non-value-added time can really help improve flow.
But, it's wrong to say only Six Sigma is used to improve the value-adding steps. Lean has methods that are used to reduce waste AND improve the actual work.
Lean uses "standardized work" as a concept and practical method to improve the way the actual work is done - including, yes, the value added steps.
I'm really not sure where somebody would get the idea that Lean doesn't help you with the value adding steps.