Yeah, in a monopoly situation I see where you're coming from, but Apple has very healthy competition from Android. It's not currently in Apple's interest to hobble 3rd party apps; quite the opposite.
What I think it may be different in Apple's case is that they don't usually build apps that compete with apps from other developers like Microsoft did.
They sometimes do, though, with Maps for example.
... and Maps is precisely one of the apps that use private APIs.
the tech industry likes to change definitions to fit their viewpoints.
On the other hand, it could be used as evidence of part of a larger anti-trust case, but given that Android devices are cheaper, perform just as well, and have a strong ecosystem, that's going to be hard to pull off.
Does Honda have a "local monopoly" on sedans because Civic drivers don't usually switch to a Kia Forte overnight?
So to get back to Apple: They don't have a monopoly on smartphones, they have a monopoly on iOS app stores.
When deciding if a monopoly exists, the relevant threshold is whether the customer had other reasonable options at the time of entering the contract, and in the smartphone world they unequivocally did have other options.