While the spec sheet does say that, it's not actually true. The PMS5003 is a truncated nephelometer (https://amt.copernicus.org/articles/15/655/2022/) that measures the total scattered light from all particles with diameters less than ~1 µm. This single scalar value (proportional to the total scattered light, as collected across a wide viewing range) is then correlated to a reference measurement of PM2.5 and typically correlates quite well in most urban areas where PM2.5 and PM1 are highly correlated. For more info on the difference between particle counters and nephelometers in this context, see https://amt.copernicus.org/articles/13/6343/2020/.

TIL! Thank you for the detailed explanation and links to additional context.