https://www.statista.com/statistics/272307/market-share-fore...
ios isn't a monopoly, there is always the other option. The option that has six times the marketshare.
again, this is literally about shutting down any possibility of a company ever offering a "walled garden" model. It's not that apple is dominant in the market - they're not. You have many many alternatives to using their devices, they're about an eighth of the market. But even that eighth cannot be allowed to exist, that business model has to be shut down permanently.
If you consider selling apps to ios devices to be its own market, then yes you are forced to use Apple. If you don't consider that to be its own market but only a part of the actual "phones and tablets apps" market then no you are not forced to use Apple.
I believe because of the size of their userbase that apps for ios devices are a market on their own, therefore "you are forced to use Apple" applies. Here the EU seems to agree.