Private/secret APIs in DOS/Windows were a prominent part of the US and EU antitrust lawsuits against Microsoft in the 90s/00s.
It mattered because Microsoft had 95% of the operating system market at the time and was using its monopoly position to take over the web, even after signing a decent decree with the US government.
The current web monopolist (Google) was coincidentally founded 2 months after the US antitrust lawsuit against Microsoft was decided (july - september 1998).
Similarly meh results with US vs Google two weeks ago.
I don't think that's a credible argument. Apple, at best, has about 55% smartphone marketshare in the United States--and significantly less in most other countries.
Remember, having a monopoly isn't itself illegal; it's using the monopoly to disadvantage competitors, especially in emerging markets, which was what the Microsoft case was all about.
I don't think there's a legal justification for suggesting that Apple creating a private feature only they can use--for now--gives them unfair advantage in the market.
I wouldn't be surprised if Apple makes it a public feature in a future release of iOS 26.