The sentence is taken from Volume No. 5 of「 波よ聞いてくれ！」
The radio-host Minare is interviewing the spokeswoman of an underground cult. When she introduces her as a member of a cult the spokeswoman claims that she is working for a public-service corporation. After discussing this matter a bit Minare says the following:
What I was wondering about is the 「そのスジの人」 .
There are several definitions and posts I found about it, however they somewhat disagree when it comes to the nuance/implication of this phrase:
These definitions here seem to understand the expression as rather neutral, while the definition of 「筋者」, which popped up when I was searching for the meaning of the expression in question as well, clearly states that it is more or less only used when speaking about criminals. (zokugo-dict.com entry)
From the context of the Manga it is pretty obvious that mentioned spokeswoman is associated with criminals (and Minare knows that).
So, what exactly are the nuances of this expression? Is it a neutral expression used to somewhat hide details about a certain person or is it strictly used for people associated with criminal activity?