Apple has already been successfully prosecuted under the Sherman Act for antitrust behavior despite not having a monopoly on devices. https://en.wikipedia.org/wiki/United_States_v._Apple_Inc.@Silent_Buddha quoting quotes quickly falls apart so a non-quote response. How does the Sherman Act apply? Apple is selling a bunch of devices to a very small number of people in the mobile market. The issue here is not that Apple is seeking to establish a monopoly, or even control the market, but that Epic (and others) do not want to pay Apple (and Google) a 30% cut for transactions. Nor does Apple's position restrain competition in the market because Apple's customer base is tiny and the market is massive. I would agree that from a devs position, it should be much easier to asset pressure on Apple but withdrawing App support, driving Apple customers to other platforms.
Remember Google also take a 30% cut on transactions made through their payment system and Android has what is much closer to (but not legally) a market monopoly. Not wanting to pay something is not a basis for convincing a jury that an act is anti-competitive. I don't like paying as much as I do for gym membership but it's not anti-competitive. If I start selling cars and Bob down the road sells them cheaper that's not inherently anti-competitive. To apply anti-competitive behaviour there has to be a specific, and citable, breach of one of the provisions of the three Acts.
But fundamentally, if Apple were abusing anti-competition law, it would be the FTC to investigate and prosecute, not Epic. Epic are filing a civil suit. If they genuinely Appel was in a position of use of anti-competitive behaviour they would file complaints with the FTC and EU and have the Government Regulators force Apple to change.
What it's worth, I think taking 30% on the value of transactions is absolutely outrageously, whether it's iOS, Android, PSN or Live transactions. A 30% cut of the value of what could be years of work and tens of millions of man hours, just for processing a payment? WTF.
And the EU has opened two antitrust investigations into Apple despite them having even smaller market share than in the US. https://www.engadget.com/apple-pay-app-store-eu-antitrust-investigation-launch-112808905.html