From Merriam-Webster, a well-known English dictionary[1]
A hacker is:
"a person who is inexperienced or unskilled at a particular activity <a tennis hacker>"
But this can't be the definition that these people are citing - Because, on the contrary, they claim to be 'experts'.
So, the next (apt) definition is, w.r.t computers:
"an expert at programming and solving problems with a computer"
But they don't program either. And citing just 'solving problems with a computer' is a very vague definition for a hacker. By that definition, even a 'data entry guy' would become a hacker.
so you see, the term hacker has nothing to do with marketing. It would be more apt if people call themselves 'marketing experts', rather than just 'growth hackers'. Sometimes, it could be mis-leading. For instance, observe:
TOM: Hey, what did you do at your previous company?
HARRY: I used to hack stuff.
TOM: [Imagines: Wow, this guy must be a genius! wonder if he could find out Lena's Facebook password!]
* to cut, notch, slice, chop, or sever (something) with or as with heavy, irregular blows
I think it's quite fitting