why something like this should be in the art thread? I wonder why

Well, who knows? it does have some sort of art in the thread, however, both hampa and thorn posted in the thread and show no signs of moving it. As a result, I'd say it's safe to assume the thread is meant to be there for some reason. So i wouldn't worry about it.

so something like that can be post on the art?

I think you are unaware of the rule called "staff can do whatever they want because they have power to do so".\\
/serious
Most of these threads are just for fun, they most likely go to hall of shame after some time.