Yep, they'd stated a few times that they don't accept new contributors. I think the exceptions are only when they come across someone who brings something different to the site, something they don't already have. I've tried a couple of times as well to get in there, with no luck.
It's an interesting business model. I guess they know they can't compete on quantity, so why bother even trying. Instead they seem to be especially selective, although not strictly on standards of quality, technicality, originality, etc. More on diversity, keeping a smaller roster of artists of varying styles.