a) There are no rules that mandate that a "general computing device" must be open in whatever arbitrary way you've not defined.
b) There are no precedents that state that a platform must have competing App Stores or make them freely available for anyone to publish. In fact the opposite is the default in companies today e.g. PSN, XBox, Tesla, Shopify, Salesforce.
If you have case law that contradicts this please by all means provide it.
What does it being a general computing device have to do with anything? Is it the only general computing device available? PC's and Androids exist, how is Apple in anyway a monopoly when they don't even have control of the market?
The are abusing a dominant position (as the only seller of iOS devices) to cross subsidise their software business and muscle out competitors in the services space (Apple Music vs other music services). That is a fairly clear breach of antitrust rules.
The prohibition of third party transactions or third party App stores are less clear to breach, but there is a case to be made...
Pandora, Spotify, Tidal, and Amazon music are
all available on the App Store - they are not muscling any competitors out. If they were those apps would simply not be available.