> If a LLM can transcode correctly, then it should also output 100% correct C code

An LLM can't (currently) transcode correctly in a vacuum. It needs tight guardrails to keep it on the straight-and-narrow (such as an existing conformance test suite that is extremely comprehensive).

The value of transcoding to Rust specifically is that the compiler gives you a pretty substantial set of guardrails "for free" - in a C port, your conformance test suite would also need to test every aspect of memory safety and fearless concurrency...