The author fails to make his point quite badly. Of course if everyone's self assessment was random the bottom quartile would overrate themselves! And that would be half of the Dunning-Kruger effect and we could truthfully say "the bottom quartile of people overrate themselves"!

The other part where those at the top have a better idea or where they rank noticeably does not come out in his toy example.

Honestly, he comes across as not having the slightest understanding of how people interpet those graphs...