First, I love the word "slew." You so rarely get to use it in a sentence. Second, this makes an excellent point, though I'm not going to discuss it in detail until I see the final version in POQ, in case there are some changes. That said, I offer the following from the discussion section of the pdf above that discusses what the author found:
The first is that scholars may have overlooked evidence that the public as a whole is generally more competent than believed by not including citizens' local political knowledge in their work. The second is that, even if the public as a whole is not more competent, evidence of specialization by citizens across contexts may indicate that certain groups of citizens are more (or less) competent than previously thought.The argument here is simple: we need to reconsider our assumptions on who tends to be competent and who tends not to be competent based on previous studies that rely largely on national data. "Different citizens are knowledgeable about different matters based on relevance, accessibility, and aptitude," writes Lee Shaker. After statistically controlling for a number of factors, "neither black respondents nor women knew less about local politics than their white, male counterparts - though they were less knowledgeable about national politics." That's a significant, and fascinating, finding. Plus, the advantages of education, so prominent in traditional national surveys, appear less important when examining local knowledge.
While a single study won't overturn everything we thought about the predictors of political knowledge, this one does raise some substantial questions and, best of all, challenges some assumptions.