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.
Where do they claim that? Do they say which kind of sensor they're using? Electrochemical? Metal-oxide? These sensors usually require advanced periodic calibrations, you need to correct for temperature and humidity zero and sensitivity dependence, model sensitivity drift as the sensor ages, take into account cross sensitivities to other gases.
I'm not disputing anything, I'm sure they do all that. I'd love to read some detail about it or a third party review that did a comparison with a reference device. It would also be cool to place some of their detectors side by side and see how much they agree on the measurements.
