You don't have to be a monopoly to have anti-competitive behavior, like disallowing competitors to compete with you in a market that has sizable market share, even if you own that market.
For example, if they banned any non Apple apps in the Apple App Store, there's still ~60% of the market in other stores (Google Play/Windows Store), but that still isn't allowed and is very anti-competitive.
For example, if they banned any non Apple apps in the Apple App Store, there's still ~60% of the market in other stores (Google Play/Windows Store), but that still isn't allowed and is very anti-competitive.