"Take them at their word"

Who? Has there been a survey of contributors?

"Genuinely think that coreutils would be better if it were written in Rust"

I feel like the skill-level of the contributors is high enough that they would not be so naive.

Programs in different languages can compile to the same machine code. Any advantage would be in language constructs. But if you already have an existing C implementation what advantage do you do from a Rust implementation?

I personally write in 3 languages: Rust, C++, and Fortran ( or rarely SPARK). I don't port my code across languages, because there is no advantage. If I wanted it better, I would work on my existing codebase.

Porting really only helps if the original language was hindering development, deployment or runtime. These arguments don't really hold with C, a fast, low-dependency language that is more widely used than Rust.