writes "demoscene" all across the scene
            ^^^^^^^^^             ^^^^^^^^^

Which scene is that? Is it the scene about demos? A "demo" "scene" if you will?

"demoscene" comes from demo + scene. "demo scene" and "demoscene" refer to the same thing. The latter is a more common spelling. We can also write "the scene" (meaning the demoscene) and "sceners" (meaning demosceners) and it's fully understood.

Alternative spellings of words don't have to be dominant or even popular, they just have to be understood. If you try and force someone to use a specific spelling even if you understood them with the spelling they used, that is somewhat jerk-like.

https://en.wiktionary.org/wiki/demo_scene

    Alternative spelling of demoscene.

https://en.wiktionary.org/wiki/demoscene

    The informal computer art subculture that produces and watches demos (audiovisual computer programs).

    Etymology: from demo + scene

https://en.wiktionary.org/wiki/demoscener

    A participant in the demoscene

https://en.wiktionary.org/wiki/scener

    A member of a scene, especially the demoscene