Android/iOS market share? Duopoly really. Call it a monopoly? "How is it a monopoly when..."
What they mean. What is always meant, is, you have a market position so significant, that you can use to get an unfair advantage on competition. That's what this is about, that is what all of these usually are about.
But, it gets sidetracked by misuse of words that should be understood from context.
Antitrust, isn't exclusively about monopolies abusing their market position. It doesn't care that much about whether it is one actor abusing it's position, or if it's two... Or three.