exactly what is stated. it is demoscene, and people been calling it such. https://www.pouet.net/ calls it demoscene, https://scene.org/ calls it demoscene, and so does https://assembly.org, which existed for at least as long as the demoscene been a thing.
so something that is called "demo scene" perhaps may designate other matter. the same way copyright is one word, and not "copy right", which is semantically diffrent thing.
it is not about what HN folk decides to call things, really. and written as "demo scene", well it took a second for my eyes to pattern-match the title.
English compound nouns can be open, hyphenated or closed. What is acceptable usage changes over time. Some say that compound nouns "harden", e.g "file name" became "file-name" became "filename", and they all mean the same thing, but currently "filename" is the most common spelling. "Foot ball" was acceptable in the 1800s but only "football" is acceptable now.
Your example, "copyright", has only existed in this closed form. "copy-right" and "copy right" are not recognised.
On the other hand, "demo scene" is a recognised variant of "demoscene". It may look as wrong to you as "color" looks to me, but it's recognised.
Want a citation or two?
The Death of Kefrens (1991):
https://ftp.scene.org/mirrors/amigascne/Scrollers/K-Groupste...
How about "Break's Amiga Demo Scene Collection"? http://arabuusimiehet.com/break/amiga/Or this 2007 interview of Fiver2? https://www.hugi.scene.org/online/hugise03/hugi%2033%20-%20d... "How do you think Debris will influence the demo scene?"