Is it? I wrote a pure rust iOS app recently. It uses native controls, and looks and feels great. iOS and iOS-sim are both very well supported targets by the rust compiler.

I do not feel so.

Not only Rust does C API, but Objective-C the way it allows to call Apple platform and call Apple compatible functions back.

Also well integrated Rust programs(with platform calls and low level hardware access) are well compiled without Apple SDK.

3-4 kicking Rust solutions are easy findable in this area.