Antitrust law is about much more than just monopoly. Apple doesn't have one, nor do they need one to be prosecuted for anticompetitive behavior and abuse of market position. If you're not familiar with the difference between the Sherman Act and the Clayton Act, this is a good primer: https://www.britannica.com/event/Clayton-Antitrust-Act
Apple is about to get bitten hard, not for being a monopoly, but for anticompetitive business practices.