Nobodies trying to overturn the concept of closed systems or trying to get access to source code.
Where Apple are vulnerable on anti-trust is their over reach concerning revenue from thin clients.
Fortnite is
not a thin client, so their claims are mostly bullshit, but Apple could easily have avoided this by not being so greedy and just allowing complimentary purchases to be valid.
But where Apples defence about providing the underlying tech so needing to earn a cut of any purchases fails is where app developers are providing whats basically a compiled webpage to access data that is wholly seperate to anything provided by Apple, where an iDevice is just a platform to access that data or services.
Literally nobody except Apple is pushing for a cut of all extraneous revenue streams, which is the basis of the lawsuit by Spotify, the basis for supporting statements by Facebook / MS / Match Group et al, the reason Valve had to make a special IOS specfici version of Steamlink removing any storefront, the reason XCloud Gamepass won't be there, and
the recent embarassing u-turn about WordPress.
It's like Comcast demanding a subscription fee cut for people buying things on webpages because they provide underlying network infrastructure to allow people to use the internet in the first place.
It's insanely greedy, and frankly indefensible rent seeking behaviour that is going to be very difficult to persuade a jury as to why a business built using nothing to do with Apple should have to share revenue just to allow existing customers access on an Apple device.