Exporting a stable C ABI/API in no way requires writing the implementation in C. See Android's NDK for a rather widely deployed example. All the APIs are C, yet none of the implementations are C. Same thing works great in Rust, too. You can trivially export C from a Rust implementation.
My comment acknowledged what you state, but then went on to point out that it requires adding a tool chain to compile Rust or C++, neither of which are trivia and which may not exist at all on the target architecture.
