The article may not take into account the possible effect of knowing about the Dunning-Kruger effect (or cultural sayings that goes in a similar direction) may bias measurements. Before it was widely enough known it was not a factor.

Also, negative knowledge comes in two flavours, what you know that you don't know and what you don't know that you don't know. There it may be ground for that effect, but also changes in culture may affect that, specially with exposure to internet/global culture and attitudes, that may make you more aware of what you don't know, and stories of success/fail for taking the wrong approach.