@phoenix

No, I’m not missing that. I am saying that places with different criteria for testing will have different relationships between positivity and prevalence, so comparing positivity rates between such places — as the author does — is a meaningless exercise.

All we know is that places with a higher test-positivity rate likely have a higher prevalence rate,

We know that‘s true for places with similar testing criteria, but not for places with different testing criteria. Thus, the difference between positivity in the US and S. Korea may be largely explained by the different criteria for testing. It may also be largely explained by lower prevalence in S. Korea. We just can’t tell from the positivity rates themselves.