Dath Ilan is a parallel Earth on which human civilization has its act together, in ways that actual-Earth does not. Like actual-Earth, citizens of Dath Ilan sometimes take standardized tests, both to figure out what sort of jobs they'd be suited for, to make sure that its educational institutions are functioning, and to give people guidance about what they might want to study. Unlike Earth's, Dath Ilan's tests have had a lot of thought put into the choice of topics: rather a lot more economics, rather a lot less trigonometry and literature. Topics are selected based on cost/benefit; something that takes a long time to learn would need to be a lot more useful, or have major positive externalities to more people knowing it.
I want to create a test, that will tell people what topics they ought to learn, and enable people to make their knowledgeability legible.
What topics belong on it?
Partly agree with your criticism of the quoted claim, but there are two things I think you should consider.
First, evaluating tests for long-term outcomes is fundamentally hard. The extent to which a 5th grade civics or math test predicts performance in policy or engineering is negligible. In fact, I would expect that the feedback from test scores in determining what a child focuses on has a far larger impact on a child's trajectory than the object level prediction allows.
Second, standardizing tests greatly reduces cost of development, and allows larger sample sizes for validation. For either reason alone, it makes sense to use standardized tests as much as possible.