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.
Like you said, the US helped Carnegie stage actual massacres before the case was made that they were a monopoly. It's entirely feasible that Apple has and abuses monopoly power, but the politics of implicating them don't align yet.
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.