They do get to say that. Apple has a minority of the market and wouldn't run into legally-justifiable antitrust problems.
I said legally-justifiable antitrust claims. Apple has a minority of the smartphone market. You can make anything into a "monopoly" if you define it narrowly enough but that's not legally justifiable in any way.
Unless you want to legally force every company to surrender their source code (which I enthusiastically would support), there's no legal basis for Apple not being able to have their proprietary feature phone locked down.
Though if you'd like, you can look at Keurig, which does limit what you can put into it to "only Keurig products" or one of the millions of other things that limit what you can put into it.
Whatabout...