"There's a cheat-code in US antitrust law, one that's been increasingly used since the Reagan administration, when the "consumer welfare" theory ("monopolies are fine, so long as the lower prices") shoved aside the long-established idea that antitrust law existed to prevent monopolies from forming at all."

Antitrust law governs anti-competitive conduct, not the formation of monopolies (cf. "trusts", as they were known at the time the Sherman Act was passed). There is is nothing illegal about forming a monopoly. The question is whether the defendant used their monopoly position to engage in anti-competitive practices.