Presumably they are able to do it because they own the rights and can grant a non-GPL license to Apple for distribution.
This seems to me to still be a “nobody can fork this [and still have a viable iOS app] but us”.
I remember pre-COVID trying to validate the popular claim that the App Store terms were incompatible with GPLv3 but being unable to do so. None of the provisions that were originally called out by the FSF were in the App Store terms anymore at that point. Certainly nothing I found in the terms at the time indicated any incompatibility.
Maybe the European Union comes to the rescue... (for Europeans)