It's an error. That chart lists forms according to the rank you test for after learning that compulsory form. Which makes no sense, really, because that means you can test for 10th Dan after learning Illyo. But no such test exists. The Kukkiwon 10th Dan is an honorary rank, almost always awarded post-mortem. So obviously there is no compulsory form.
Whoever wrote it switched from "what rank do you test for after learning this form" to "what rank do you need to hold before you learn this form" at 6th Dan.
In our branch of the MDK, we fixed this confusion by shifting the forms down a notch. You learn Koryo to get your 1st Dan, and so forth. So basically, if you have a Dan rank from our system, you have already passed the KKW test for one rank higher.