The Internet and the Web are not the same thing. Apple absolutely gatekeeps the ability to run non-HTTP internet connected apps. Your statement that Apple is not a “gatekeeper to any part of the Internet” is simply false.
So, since UDP is allowed, I'm curious as to what non-HTTP traffic Apple bans apps from using.
I hope the DOJ adds this to the list of Apple's transgressions, along with green bubbles making Android kiddies feel bad.