Agreed. It's not a "foreign" function if everything is in WASM. All the VM sees is WASM code; it doesn't make a difference if it was originally written in C or Rust.
"Foreignness" is an incidental property, the actual goal is interoperation. Being able to interoperate with C code from within the sandbox is both useful and something that BEAM doesn't do. In the meantime, there's nothing preventing anyone from doing regular FFI from within the part of the Rust program that lives outside of Lunatic, if for whatever reason the sandbox is insufficient.