Lots of things are dangerous even at concentrations measured in PPM. For example, the level of Phosgene that’s “immediately dangerous to life” is 2 ppm: https://www.cdc.gov/niosh/idlh/75445.html.

Maybe the point is that this 20 ppm quickly turns into less based on further dilution. But there’s a lot of analysis required to support the post’s assertions that the author just skips over.