So, "at least one operating system programmed in Fortran" doesn't counter "Fortran hasn't been really used for systems programming", if that operating system is some irrelevant research project, or (as in here) a commercial niche system, that maybe ever reached 0.00001% of the OS users space.

The first section of the linked article says that Fortran "was an odd choice from a pure computer science standpoint", that they "developed hardware optimized to run FORTRAN", and that large parts of the OS (in particular, the kernel) were eventually rewritten in other languages.

This is, in my opinion, a great example of how Fortran could not compete with C as a systems programming language.