"unnatural" or "artificial" have a very clear meaning: made by the intervention of humans in ways outside our base anatomical functions (so excluding babys or spilled blood). This intervention can be mechanical (Stonehenge), or biological (breeding animals to become more useful to humans), or chemical (synthesizing oil from plastic), or a complex combination of all of them (whisky); it is also transitive: anything created by an artificial object is also artificial.
The fact that a naturally occurring thing (human beings) can create artificial things is not surprising under this definition.
The definition can also be theoretically extended to other human-like agents, like hypothetical aliens. It hasn't been practically very necessary so far.
Edit: I should note that this is the sense of "artificial" or "unnatural" that is used in the context of the article. There is a secondary meaning, used in phrases such as "artificial sweetner" vs "natural pesticide" that I don't think stands up too well to serious scrutiny.