You're right - and Epic isn't arguing that they have to be let on to the App Store, except in the case where there's no alternative. Ignoring the fact that Fortnite isn't actually a platform that allows you to submit skins to Epic (and as such isn't really comparable), you absolutely can make your own skins and 3d models and content and sell them somewhere else, you just can't in Fortnite.
And before you say "oh but they can just go to Android" - no, they can't, because Google Play has similar restrictions and while making a separate store is possible it isn't practical due to technical restrictions Google imposes on the OS.
> I also can’t sell any game I want on their platform. I have to follow their rules, get approval? Etc.
I've already addressed this. On iOS it is impossible to sell an app outside of the app store. If you're denied from EGS you can just go to Steam, itch.io, GOG, or host a website yourself.
> Not to mention video game exclusives.
This is a completely unrelated issue. The court case is about the rights of a developer and their relationship to the app store. This argument is about the rights of a consumer and I do not see it as even remotely relevant.
> The only reason we are talking about it is because it’s a popular platform.
You're right - the platform being popular is what gives Epic's argument merit. It makes Epic less able to ignore the app store if they want to go after the mobile market. Obviously if the platform wasn't popular then no one would care and this case never would have happened.