“Who the hell” writes an OS in assembler in the 80s? Uh, some of the utilities are in C, but compilers were slow and generated slow code back then, and it was quite noticable on a slow machine. When every byte of memory counts, you often need to hand-optimize.
You’re thinking of Shit Jobs, the FruitCo charlatan. Gates’ pancake sort algorithm held the speed record for 30 years.