I'd file that under "domain knowledge". Not only marketers need that though. If I am a coder and develop software that is used in fishing (and thus aquire knowledge about this domain), what does that make me? A maritime hacker?
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.