This is the second time that I see Amazon mentioned as getting preferential treatment. How? AFAIK Apple differentiates between payments for physical goods and services and electronic ones. It allows for external payment mechanisms for the former, but not the latter. That's why you can buy stuff in the Amazon app, but not in their Kindle app.
One can debate whether 30% is 'fair' at this point in the iOS ecosystem's life, but I don't think Epic has a legal leg to stand on - they simply violated a contract. Some have made the argument that Apple frequently changes the contract with developers having no recourse to accept the new terms. If the changes are, indeed, more restrictive on developers, that might be a good avenue for Epic to pursue. But I'm not aware of any such changes - the only change I've heard about was to to the benefit of developers: the change from 30% to 15% Apple fee after one year of subscription.
The preferential treatment I was referring to is that it is widely believed that Amazon Prime Video only has a 15% Apple Tax which differs from “Apple’s Rules”. There were emails between Apple and Amazon that were made public where this deal was detailed.
My point is simply that if Apple wants to continue saying that they treat all developers equally, then they need to define a criteria that is transparent for all developers to achieve the 15% Apple Tax.
The criteria can be lofty, and maybe only a handful of developers will achieve it, but it still needs to be defined. I think it can be reasonably assumed that FortNite would hit whatever criteria they defined.