I don't think the law defines monopolies so narrowly that it's useful or interesting to narrow it down to "devices compatible with Apple's Ax series chips."

Back in the MS monopoly days, there were a ton of things people wanted to do where they had no choice but to buy Windows if they wanted to do them. Today, there is very little that people do which requires an iOS device. Apple has many closed features (the App Store, iMessage, iCloud) but open equivalents are available. Almost nobody is buying an iPhone because they need an iPhone in particular to do something. They're buying iPhones because they need a smartphone, and they prefer an iPhone.

I am very much not a fan of Apple's approach here, I just don't see anything illegal about it.