When I turn off JS, it shows a loader instead of the terrain. Is this really CSS-Only? I mean it's still a high achievement even with JS, but was expecting it would also work without JS. This one, for example, truly works without JS https://benjaminaster.com/css-minecraft/ .
I think what's intended is that the completed and downloaded solution doesn't require any javascript.
Build something then hit the Download Code button - that packaged HTML solution doesn't require any javascript to render locally.
I'm assuming it's the render engine that is in pure CSS. You could display a static map in CSS but things like the tools to modify the terrain definitely need JS.