Tiny binaries probably relied heavily on the native OS's system libraries.
First to set the video mode, and then to set up a timer used to progress time.
There is even more debate in 4k. After all, most rely on graphics drivers that take hundreds of megabytes. But the thing to understand is that in any case, the intro ships all the code that produces the sound and image. The OS is just an abstraction layer. The exception would be fonts and MIDI instruments, that can be stored in the hardware or OS.
But not all intros have text, "Memories" doesn't. And many intros do their own sound synthesis, though in PC 256 bytes you are usually limited to MIDI or to that horrible buzzer.
Anyways, great job. I was there during the compo, it was epic, with everyone double checking the executable size, even the old guys who have seen it all. You got my vote BTW.