Correct me if I'm wrong here, but I'd expect that iOS being an open platform is neither necessary nor sufficient for that. As I understand it, Epic could allow users to load free custom skins today, even though it operates on a closed platform. And conversely, even if iOS were an open platform, Fortnite could remain closed-source, and I'd expect Epic could take measures (hash-validating assets on launch?) to prevent side-loading of cosmetic items.
On a traditional PC, or on a Mac with System Integrity Protection disabled, there isn't a lot Epic could do to prevent installing mods. The user can inject whatever code they like into the Fortnite process. They can rewrite data in memory, and even control the kernel if it comes to that.
Epic could try to make life difficult for modders, but a lot of PC games have tried that over the years, and it generally doesn't work when players are dedicated.
