I know a guy about same height and weight that is naturally lean like 6% -8% bf years around. However, this guy I know has a very hard time puting on muscle....he can stay lean but def hard to put on muscle he even told me that. He doesnt juice as per injectibles, but he takes anything that would boost test that he can find legally and help him get preped for a show.
Now like you said he may not juice per say but there are plenty of other stuff there that will help with the prep. Now this guy I know does natty shows too....is he natty or not I guess it all depends what compounds he takes. If these compounds alter your system in any way then I'd say he is not natty anymore.