That article says that the US military has the largest single install base for RHEL in the world, but that was about 15 years ago, I don’t know if that’s still true.
Apparently back then the US nuclear sub fleet and its sonar systems also ran on RHEL.
I suspect lots of military hardware runs some form of *Nix or BSD type system. Many embedded systems run some *Nix type OS, and a huge portion of the developed world’s weaponry is smart, so it it full of low power embedded systems and custom SoCs.