It's about the restrictions they put in place on those who have bought their products. They make it extremely hard to leave their ecosystem, overcharge for services, and prevent interoperability with other products.
They're only just now allowing RCS to try getting out ahead of these lawsuits. But they still don't allow side-loading (forcing developers to use their store so they can charge 30% on all sales), don't allow third-party app stores, and don't allow third-party browser engines (forcing everyone to reskin Safari).
Plus, in the US, Apple has 55% market share as just one company. The next highest is Samsung at 25%.
So yeah, they can play monopolistic games.
It would be far from "slavery" in the typical sense.
If this happened, those people would get food, housing, healthcare and job training while getting paid for it.
Though I'm definitely not in favor of conscription because how can you call yourself free if you're required to do involuntary military work for the government.