I don't know if it's significant from a legal standpoint, but for as long as there's been an App Store, it's been a walled garden. Yet Apple still became the most profitable phone manufacturer, despite that. It's not like they took advantage of their position in the market to force some new anticompetitive/anti consumer behavior the same way Comcast might.
Most monopolies are only sued (or considered in anti-trust) when they are in a dominant position. In this case, there are two dominant phone platforms... Apple's iOS (iStore only), and Android (multiple phone vendors, Play Store and options to load whatever software a user wants).
In the Apple case, they've taken whatever legal means possible to stop side-loading and alternative stores, dropped apps that try to sell via other paths. They have a vertical stranglehold on the platform, and are in a dominant position.
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.
