This is not ‘autocorrelation’, it is regression to the mean. I find the article unclear and imprecise.
For those interested in a better overview of the Dunning–Kruger effect, I recommend this short article by McIntosh & Della Sala instead:
> in the academic literature, it has been suggested that the signature pattern of the DKE (Figure 1A) might be nothing more than a statistical artefact. In a typical study, people’s tendencies to under- or overestimation are analysed as a function of their ability for the task. This involves a ‘double dipping’ into the data because the task performance score is used once to rank people for ability, and then again to determine whether the self-estimate is an under- or over-estimate. This dubious double-dipping makes the analysis prone to a slippery statistical phenomenon called ‘regression to the mean’.
https://www.bps.org.uk/psychologist/persistent-irony-dunning...