For "precisely" read "correctly". Unless the implementation language is exposed to users of the implemented language, it doesn't matter what the implementation language is. gfortran is written in C and C++, but that does not mean Fortran programs are written in C or C++.
Okay, I think what I said was clear enough, but if we're splitting hairs, I'll say: that's a bad comparison because gfortran is compiled to machine code. When you run a gfortran program, you're not executing anything that was written in C.
When you run e.g. Python, the runtime written in C is indeed being executed. If nothing else in the world was written in C except the Python runtime, IMO it would still be fair to say that C is widely used. A security flaw in C compiler could potentially cause a vulnerability in any Python program.
The question was "is Scheme used for anything in production?" and it is correct to say "yes it is used for HN" even if HN is not directly written in Scheme.