>iPhones don't segment anything, yet they are locked tight.
Of course they do. There's market segmentation between iPhone tiers, between just using the iPhone as a computer (connecting it to a monitor and running macOS on it), differentiating iOS from the much less tightly maintained Android ecosystem (for which "walled garden" is a feature felt as "less hassle, less malware, more secure, mostly just works"), and several other things.
It absolutely is a design choice. Apple wants to control all the code that runs on users' devices. Market segmentation is merely a side-effect.