They are pointing out that Apple allows some apps certain privileges that others do not have access to, and that Apple will bend their own rules based on arbitrary decisions.
As I understand it, that's not
exactly true. Any dev can use private APIs, if they know about them. Yes, Apple will generally reject your app if they find out, but it's not as though access to private APIs has been been magically be
given to one company alone. I'd imagine there are actually lots of devs flying under the radar and using private APIs (I've definitely heard of audio devs using private APIs on occasion), and probably quite a few instances where a dev made a good argument to Apple about
why they needed a private API, and managed to get approval. If it benefits the user experience then Apple probably wouldn't be too fussy; they'd just warn you that it might change at any time and break your app.
The main reason for private APIs remaining private—again, as I understand it—is because Apple wants the option to change them at any time, without having to go through the whole process of deprecation, updating the public the API, updating documentation, and so on. Public APIs have to be maintained, consistent, and syntactically identical across versions. Not so with private APIs, which makes life way easier for internal devs.
Epic's little diaper-filling spat with Apple is very, very different. They want to be exempt from the basic rules that everyone on the App Store follows.
BTW, I'm willing to be wrong about this stuff, since these are just things I've heard during my time as a developer, and I don't have the time or inclination to run around fact-checking this s*** for the purposes of playing patty cake with garden variety MR trolls (which
isn't to suggest that you're one such troll, btw... just covering the bases... you're just adding a point... I get it).
EDIT: Which is all a convoluted way of saying that Epic is playing politics, for their own gain, and that's it.