I used to write for wattpad too! But I decided I liked this platform better lol. It's more personal, and the books here are better quality than wattpad because it's not overrun by kids.
Yeah, premium stories are definitely more emphasized on the site. But technically anyone can become a premium story if you but the work into it. I emailed one of the workers and they said that if you have an almost complete or complete story, you can sent it to them, and if their editing team likes it, they will accept it as premium. I believe its a really good way for indie writers to shine 
Plus, I looked into some premium novels, and the writing levels can range from beginner to professional writing. So if you have a good plot and most of your story done, I believe that any of us can do it 