Yeah, it's a web browser in essence. But the UI, scrolling behavior, etc. are all handled by the app rather than the website. The result is often a much cleaner experience, especially compared to the average website's mobile web experience. There's also often extended functionality, like saving entire galleries of images, saving threads, etc.

What I'd imagine Artstation doing is releasing a "generic art showcase app" (or exposing APIs to let 3rd parties do so themselves), where users can manually specify www.artstation.com. The app would provide all the features that the Artstation app did.