The choice to display anything under 35 μg/m³ as "good" does not seem unreasonable at first glance. Indoor PM2.5 can often be higher than outdoor so that the yearly average will be pushed down by time spent outside and there is no need to have <5 μg/m³ inside too. Also, by the author's admission the unit also measures particles larger than PM2.5 which are less harmful. There's no point in having a unit that will flash orange or red for almost everyone almost all of the time.