I think that "in-person usability testing" refers to the process of actually watching a user use a product. I agree with you that these aren't general truths, but I don't see where the author said that; the author said that these are "general findings". Also, they are general findings for a specific target market: "lower-income users who access the Internet at least once a day on a desktop at home or work, or on their phones". My guess is that anyone who visits Hacker News does not fall in that target market.