I am a security professional. I would personally not care less to make the distinction, as both are very generic terms that are used very liberally in the industry.
So I don’t see any reason not to call this hacking. This was not an intended feature. It was a gap, which has been used to perform things that the application writer did not intended (not in this form). If fits with the definition of hacking as far as I can tell. In any case, this is not an academic discussion, it is a security advisory or an article that talks about it.