Hence why UNIX on mobile is a Pyrrhic victory, as iOS, Android, ChromeOS rely on Objective-C, Java and JavaScript runtimes and their respective frameworks, just with good enough support from POSIX, that could be replaced by what is expected from any ANSI C implementation.
There is no Objective-C "runtime". (Well, there sort of is, but it's just a library, not a heavy-handed thing like you're thinking of). Unlike Android, iOS apps are just normal compiled ARM machine language binaries executing natively.
Yes, you have to use their APIs in order to write graphical programs, but the same is true on any OS with a GUI system.
It's possible to write iOS apps in pure C if you want to. Sure, that'd be a pain, but it's possible. Less painful and actually decently reasonable would be to write all the GUI-specific stuff in Objective-C and any other logic in pure POSIX-conforming C or C++, since you can mix all those languages freely in a project.
