Sorry to ask but, what do you and the author refer to with 'demos'?
The demoscene is an international computer art subculture that specializes
in producing demos: small, self-contained computer programs that produce
audio-visual presentations. The main goal of a demo is to show off
programming, artistic, and musical skills.
Basically, these programs are extremely small, but produce interesting and impressive visual or audio experiences. A popular example would be "fr-041: debris. by farbrausch": https://www.youtube.com/watch?v=mxfmxi-boyoFor anyone who want a very spesific example of how cool demos can be, here are some of my favorites:
https://www.youtube.com/watch?v=wXMs54NUBOI
https://www.youtube.com/watch?v=xOYpKnl1B_g
https://www.youtube.com/watch?v=OO6dtjxbHzs
Going to see these demo-competitons in Norway in my childhood was certainly a big inspiration to learn coding.