I’m also not a lawyer but my understanding is that anti-trust law covers broader stuff than just monopoly busting. There’s no reason to require Apple to be a monopoly before we start to worry that they might be engaging in anti-competitive behavior.

IMO arguing about whether or not they are a monopoly is a losing pedantry battle that misses the point. People will bring up exact market share, and what exactly is a market—honestly, some good arguments about whether or not Apple is technically a monopoly—but, who cares? They are clearly a major player and so if they are not competing fairly that’s clearly a major problem.