Smartphones are in a market, yes...? I can also buy an Android or Pinephone, or use an open source OS like Graphene. That means you have a choice not to use iPhone's garden or use Google's or none... via F-Droid style app stores.

Do you think free-markets means corporations also need to run their business operations and products/platforms like a market? Because that's not what that means. (Even if it did ideological purity is dumb)

Inversely governments forcing companies to all operate with the same business model is not on a free market.

The only thing that would prevent a walled garden from operating in a free market is if Apple et al was a monopoly/oligopoly and you had no other choices. I've already said antitrust laws for things like browser choice is one of the better solutions than the one proposed.