it is true that iphones itself isn't a monopoly (yet), but i reckon consumer protection should extend to the ability to install arbitrary software on a device they purchased, regardless of the intent of the company selling the device.
This argument doesn’t hold now that Firefox’s smaller and less funded competition manages to deliver. Being forced to use a webview does not leave you as helpless to implement features as Mozilla stans claim.
being able to block ads, vs being able to install extensions, are two separate requirements with different difficulties. In the past, apple was anti-extensions - your app weren't allowed to be able to load remote (and thus unreviewed) extensions that executed code and changed app behaviour.