They're in a dominant position on their platform, which makes since as it's their platform. What they're not is in a dominant position in their market. If you

They're in a dominant position on their platform, which makes since as it's their platform. What they're not is in a dominant position in their market. If you try to define antitrust in terms of platforms instead of markets, then every company that has a platform is a monopoly. And that is, of course, quite absurd and renders the term "monopoly" meaningless. Which is why it's not actually interpreted that way.