I'd say it's neither. Demoscene is a good example of when designers and programmers were working well together.

Deviantart was a huge platform where people would share art and designs/skins. To some extent that still exists for iOS views.

I think one thing that's different about the deviantart model was that back then apps used to have skinning interfaces. And then designers could focus on just skinning. You could argue that gnome themes are like that. I would disagree, but also skinning isn't quite the same as doing ux.

UI/UX on Mac is a lot easier than UI/UX on gtk applications or QT applications.

On top of that, I don't think most designers understand OSS. It's basically a different universe and these two world don't often interact.