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.
In the article, OP explains that they (there should be a gender neutral singular pronoun in english) are in middle of analyzing data from a certain income group who are used to getting around the web, who handle both desktops and phones. It's in the first or second paragraph!
(there should be a gender neutral singular pronoun in english)
There are a number of attempts at this but I've found Spivak pronouns (specifically the Elverson set) to be a particularly elegant solution. The easiest description is "the 'they'-form pronouns without the 'th'". Thus, your first sentence would become "In the article, OP explains that ey ([...]) are in middle of analyzing data from a certain income group who are used to getting around the web, who handle both desktops and phones."
Anecdotally, I've found most people seem to understand what I'm saying online and offline people either understand me or think I'm saying "they" and understand my point anyway. Plus, this pronoun set sounds more natural than other constructions and is easy to remember since it's keyed to a set you already know.
I used to resist using the singular "they" for one person (preferring Spivak pronouns) until someone pointed out to me that documented use of singular they for someone of unknown gender dates back to the 1600s.
And, perhaps most importantly, people will understand you when you say or write it.