I agree with the overall point of the article, but I feel compelled to be _that guy_ and point out that most of IBM's systems programming involved various dialects of PL/I, not Fortran, and they went through a bunch of different iterations on those compilers and their code generators.
Was going to make a similar comment…most systems programming was in PL/S or PL/X on 370/390 architecture (regardless of the O/S). AIX and OS/2 were mostly in C. AS/400 in RPG. There were some oddball programs in APL. And thousands of internal "tools" in Rexx.
> AS/400 in RPG
Aside from a few utilities, relatively little of OS/400 was written in RPG. Originally, most of it was written in two different dialects of PL/I. Some Modula-2 was added into the mix, and most of the lower levels were rewritten in C++ when they switched to PPC processors.