Yes, while the orks had no record of this planet, they had more intimate knowledge. That is why they were coming. More interestingly, though, this group of orks was every bit as privy to it as the Emperor.
All of this the orks had accomplished without ever having set foot on the planet.
Its Kaptain was an ork known as Zed Gorzog. Though it was an oddity in the powerhungry race, he actually prefered title of Kaptain. It might have suited him since Zed was a relative no-one in the grand scheme of things. Even in Ork-dom, his name hardly held weight....
...but he was crafty and ambitious and had those 5000 boyz behind him.