The fact that certain groups of people tend to test well (or, taken conversely, that there are so many reasons why otherwise "intelligent" people don't test well) just exacerbates this problem and I think the central claim is still reasonable: things like IQ tests aren't necessarily strong predictors of tech skills, are dangerously close to pseudoscientific when misapplied, and have enough other theoretical problems that they should probably not be utilized during a hiring process.
As to my own linguistic skills, I contend that a lifetime of reading as well as a MS (in computer science), a Ph.D (in cognitive science and education), and ten years afterwards in academics including a professorship has prepared me as a writer. Did I have some initial Potential that gave me a head start? Maybe. If nothing else, I had the head start of my general introvertedness and a love of both reading and geekiness. A short test that purports to measure a fixed potential somehow inherent in someone is going to be pretty flawed.
But my "I don't doubt" phrase was more along the lines of ceding a central point: there probably do exist certain inherent characteristics that vary between people and provide some kind of predilection or head start. I'm just not yet convinced that the scientific community has really identified them yet, or that they're really able to effectively measure them yet. Instead, we get proxies that have a very high false negative rate, especially among otherwise-marginalized groups.
Anyway, four paragraphs is probably enough here.