I disagree. An oligopoly does not need to be a cartel, but a cartel is a special case of an oligopoly, and here I'm talking of oligopoly, not the narrower case. A structural deficit in a market can lead to oligopolistic markets and exploitation without collusion, sometimes with regulation and sometimes wihtout. For more details, see (for example) here:
https://www.diffen.com/difference/Cartel_vs_Oligopoly