Someone else in that thread claims to have seen the whole episode and no credit was given. I'd be willing to bet someone at an overseas animation sweatshop is the culprit.
Sadly, I've seen this sort of thing happen before too. There's a SciFi channel movie where some fantasy artwork is seen on the wall of someone's lab. It seems someone involved with that production simply downloaded images from the website they were created for, and printed them out. No credit or compensation was given at all.