The effect exists. What this study does is explain its source. Colloquially, people would say things like: “Smart people underestimate their knowledge, dumb people overestimate it.”

What this study finds is that this is technically true but only because the distribution of possible “guesswork” is uneven. But so what?

Technically, smart people over/under estimate their skill at the same rate as dumb people. However, smart subjects are not only more accurate, they’re closer to the top of the bell curve and their underestimations are wider than their overestimations. The opposite is true for dummies. This is the Dunning-Kruger effect.