Anti trust law is federal, not state.Citing statute depends on state in this case but one could argue that apple controlling the competition of the market itself is anti-competitive. Yes, every market, physical or virtual, charges fees for use. As they should. But if a vendor in a market decides that the fees are too high they move to a different, competing market. It is a market of markets so to speak that keeps things in check.
Apple does not allow other markets on iOS. If you want to sell on iOS you MUST use Apple's market. There is no market of markets on the platform. There is on Android, Windows, and even the macOS. Steam, Epic, GOG, Origin all exist on these platforms but not on iOS in a capacity to sell software (steam has a app but you can't actually buy things through it.
That all said, I think Epic is barking up the wrong tree here. They complain primarily about the high fees and not the fact that there are no other available markets. I don't think their suit has merit on the grounds they are using. Apple should be able to set fees as high as they want. Arguing market lock-in based on their platform I think would be much more convincing in court.
In any case, this is my point. People wave their hands and claim "antitrust" but nobody cites a law.