I reached out to the author and they're using an uHoo Smart Air Monitor and will be writing a review of it in the future. Most reviews of air quality monitors seem to focus on details like the size of the screen, or whether the monitor app is easy to use, and fail to actually test the accuracy of monitor itself. Hopefully this author's review is different.

See this EPA testing table for some measures of accuracy: https://www.epa.gov/air-sensor-toolbox/evaluation-emerging-a...

Any cheap sensor (electrochemical? single electrode or even dual) sold for amateur use is unlikely to provide any useful measurement, especially if you want to draw any conclusion about concentrations exceeding safety limits like the author is doing. They are a pain to calibrate, the absolute values you get cannot be trusted, the most you can use them for is to say something about trends and spikes. "Look the gas stove is on, there's a NO2 and CO spike!", now let's get a chemoluminescence spectrophotometer to see it we should worry.