Point taken. I agree that there are a whole bunch of marketers who couldn't have come up with this idea because they don't have a basic knowledge of how these things work. Maybe I'm just fortunate enough to know a couple of them who actually do...
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.