You can already do this in the traditional way by building an ObjC shim which exposes a C API. The solution shown here just skips ObjC and talks directly to the ObjC runtime (which has a C API but is not as convenient to use as doing the same thing in ObjC or Swift).

In a highly simplified way you can think of Objective-C as preprocessor which replaces the ObjC syntax sugar with C function calls into the ObjC runtime (it's not how it works in reality, but how it could work).