There's probably one written in Brainfuck too, doesn't mean it matters in the grand scheme of things though.
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.
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.