I agree with this article, but who is to say the goal of Dribbble isn't to just showcase visual design? To join Dribbble, you must get an invite from a Dribbble user. Dribbble users invite people who have similar taste and design state-of-mind as they do. So in the end you get a gated community of designers who think and work alike.
The problem with this I see is that non-design people who wants to hire designers think that Dribbble is the "go to" standard of design, without thinking of usability problems their product[s] have that needs solving.
You nailed it. Dribbble excels at featuring and sharing solutions to visual design challenges. Icons. Logos. UI components that show effective affordance (as best you can tell from a small screenshot).
I think Dribbble is somewhat a victim of success; everyone wants in and uses it as they see fit.
The problem with this I see is that non-design people who wants to hire designers think that Dribbble is the "go to" standard of design, without thinking of usability problems their product[s] have that needs solving.