There is some C in the game as well:
It’s 99% written in x86 assembler/machine code (yes, really!), with a small amount of C code used to interface to MS Windows and DirectX.
https://web.archive.org/web/20201108105209/http://www.chrissawyergames.com/faq3.htm
Yeah, the amount of industrial machinery being controlled by ancient hardware would baffle a lot of people.
For a comparison people might relate to: There are ATMs running twenty year old versions of Windows XP.