Their compiler fails to compile (well, at least link) some C programs altogether, and in other cases it produces code that is 150,000x slower than a real C compiler with optimisations turned off (interestingly, the model trained on the real compiler's source code). That's not "not competitive" but "cannot be used in the real world". But even more importantly, the compiler cannot be fixed or evolved. It's bricked (at least as far as today's models' capabilities go). For any kind of software, not being able to improve or fix anything or add any new feature means it's effectively dead.
You could not use it in production even if no other C compiler existed.
- John Carmack embedded a C compiler and interpreter/runtime into Quake back in the mid 1990s as a scripting language! It was that efficient that it could be used in a real time 3D shooter. That's a solo effort as a minor component of a much larger piece of software.
- I've seen university CS courses hand out "implement a C compiler" as a homework / project exercise for students. It's not particularly difficult.
Sure, a modern C compiler like GCC has to handle inline assembly, various extensions, pragmas, intrinsics, etc... but like you said, all of those are thoroughly documented and have open source implementations to reference.
Similarly, the Rust compiler is implemented in Rust and could be used as an idiomatic reference for a generic compiler framework with input handling, parsing, intermediate representations, and so forth.
I would bet that those things are also true of at least one expensive commercial C compiler.