On Android you can do it via the NDK sure, but then again 90% of the OS is exposed via Java APIs, even a basic thing like opening a file requires Java, if you want to do it in a portable, certified Play Store way.
And on Windows, .NET MSIL or COM are much better ways to expose libraries.
Naturally one can still do it in an old fashion way.
That's hardly an argument for using C though. Successful OSes today were started years ago when there was barely anything else to use. It takes a long time for OS to gain success. Rust is so young, that to evaluate success of an OS written in it, you'd need to wait comparatively. So time will tell.
We could say the same related to JavaScript and Web, and it doesn't make it better.
Don't mistake success due to quality and being widepread thanks to historical accidents, free access to source code and the consequence market forces of POSIX adoption.
On Windows and Android if I'm interfacing to a Rust/Python/Java (on Windows)/C++ (on Android) library I still do it by going through C.