So I ~lied~ erred when I said that there was no software other than a naked C64 involved - thank you for the heads up! I saw a talk by Raquel Meyers at a tech/art conference a few years ago, where she explained a few of her works, including this particular video. I guess I just forgot about the software because I was so impressed by the stuff she made :)
Here's a video of her creating an animation, the technique is called KYBDslöjd or Live Type In, I think all the key-presses get recorded, then the sequence replayed in time with the music.
https://www.youtube.com/watch?v=JTTh-3weZR8
The software was developed at demo parties in the Swedish demo scene.
https://17days.wordpress.com/2016/09/15/day-14-raquel-meyers...