Correct me if I'm wrong, but this used PCR testing to determine the number of _currently_ infected people. It says nothing about the number of people that _were_ infected, but fought off the virus and are now potentially immune.
It’s only after the rate of infection stayed the same or falls for a month+ that you need to adjust for such people.
I don't think this is true. I get the flu shot every year and have never had a strong immune response to it, yet I'm still immune to the major flu strains each year.
Point being, it's possible for your body to create long term immunity to a virus without suffering from a severe infection.