> So, it's interesting to me, that, unless they have normalized for I&I somehow which is not apparent in their raw data, the true seasonal variation in concentration is actually understated.

Biobot says:

"We normalize the SARS-CoV-2 viral concentration to a fecal indicator, to account for differences in dilution. We use PMMoV as this fecal indicator, which is an RNA virus that is commonly excreted in stool."

So they're effectively reporting on the ratio between SARS-CoV-2 and PMMoV concentration, which should be normalized for seasonal changes in dilution.