You come from a limited perspective...you know both coding and marketing and some of the fundamentals (particularly of the former) are so grounded that you can't possibly imagine a layman NOT knowing them. But trust me, many of them don't :)
Aren't these qualities needed by everyone? So why even have the term "hacker"? Why doesn't Lifehacker just call itself the "Life" blog?
So to answer your question, I don't know when are where we should draw the line when assigning titles. I don't think there are any obvious rules, and in the case of growth hackers, I think there is a specific combination of skills and insight that isn't adequately covered by just marketers or by just coders.