Based on reading the rules, it seems to me that there isn't a restriction, just a suggestion to avoid posting for our own benefit. I'm guessing they are worried about people trying to steal the art or claim later it was their own art that was stolen etc...
If you look at the portion this appears in all of the other bullets say "must" instead of please. BUT with that said, I have seen a few people post, but most have either deleted or were erring on the side of caution to begin with by avoiding posting.
I will say tho, I saw one person post 4 different submissions that all appeared to be for straight Samurai swords! Although I don't see it now so I'm guessing they deleted as well.