AutoCAD and Emacs would be literal examples of "sufficiently advanced" C programs by this definition.

My guess is that it is intended to cover any C or Fortran program that includes an ad-hoc interpreter, an ad-hoc dynamic type system, or an ad-hoc garbage collector. That would include e.g. sendmail (at least an ad-hoc interpreter) and GCC (all three).

My guess is that most C programs (I don't know about Fortran) we would intuitively call "complex" contains one of the three technologies, making it harder to come up with a clear counter-example. Some might avoid the "ad-hoc" part by linking with a generic extension language like tcl or lua. Or even Common Lisp.