I think you're falling into exactly the same inaccuracy that you're trying to combat. The issue is not that everyone below the top 20% of performance over-estimate their performance, it's that over-estimating performance is strongly correlated to incompetence. Your summary makes it sound like someone who is of average competence is over-estimating their performance to the same extent as those with the worst performance, which is untrue. In fact when we talk about the DK effect almost no one is talking about the average person who thinks they're smarter than 65% of other people because that effect is almost impossible to qualitatively observe.
But that makes it sound like only people at the bottom overestimate themselves, when in reality 80% of people overestimate themselves. How about this rewrite: "People who aren't experts think they're smarter than they actually are." And get rid of the word "always", we're talking about averages, not about certainty in every case.