What strikes me about that graph is that the entire group is likely to be more skilled than the general population. The selection only of people who have the interest and means to attend higher education seems like a narrow window at the furthest edge of the true graph. So I wonder what it would look like if we included people of all education levels and social strata?
My gut, based on this article, is that it would look generally the same but with a larger spread at the lower end of the scale. But I don't think we can truly say we've disproved the Dunning-Kruger effect without a more varied dataset.