"Null hypothesis" comes down to agreeing on a prior that is "reasonable". Most of the time, that indeed means not assuming dependencies, e.g. when testing the outcomes of a medical treatment. But that's not always the case, e.g. does it seem reasonable to you to assume a priori no dependency between a person's age and their height? Does the result "People get taller until the age of 20" merit a journal article? There's a very strong correlation there after all.

As I've written elsewhere this all comes down to the prior, based on my life experience the prior "people are generally capable of self-assessing their performance" is much more likely than "people have absolutely no ability to self-assess their performance."

To state it differently, when you assume no dependencies anywhere, you've already jumped to a conclusion that is more far-fetched to me than the results of DK. Do you really think people have zero ability to self-assess their own performance? In all domains and all contexts, as this is your "null hypothesis"?