Good article, but I think there’s a distinction to be made between;

1. Designers making UIs for made-up products. I’d call that an exercise for digital artists, as you say. Which is OK—I’m not sure they’re professing to say they’ve thoughtfully developed a rounded product.
2. Designers who show the process of the resulting interface design of a real product that have had those 3 prior processes you describe (either by those people themselves or others in the team). And this really is what Dribbble was intended for—snippets of progress of interface design. And that’s OK too.

I guess what I’m saying is that I think you see Dribbble having a different purpose than you think it has.