This shows that everyone in the study (economic experts, ML experts and even developers themselves, even after getting experience) are novices if we look at them from the Dunning-Kruger effect [1] perspective.
[1] https://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect
"The Dunning–Kruger effect is a cognitive bias in which people with limited competence in a particular domain overestimate their abilities."
No, they underestimated their own abilities for the most part; the estimates for AI-disallowed tasks were all undershot in terms of real implementation time.
What they overestimated was the ability of LLMs to provide real productivity gains on a given task.