The post is mostly about OS, but their C compiler is also pretty interesting. As noted it is written in OCaml.
Their CPU doesn’t have integer multiplier (it has floating point multiplier though), so multiplication is implemented in compiler runtime library with inline assembly. The compiler does strength reduce multiplication by powers of two to bitshifts. It also needs to run raytracer, so sin and cos are implemented using taylor series. Said raytracer needs to be debugged, so printf is implemented including formatting of floating point numbers. Etc.