They've explained it before -- https://twitter.com/SolidEvidence/status/1665444605511311362... -- while they aren't confirming it's exactly one person, they've got strong data to suggest it's likely just one person, and for sure not some kind of community outbreak situation.
To complicate things, people flush more than poop down the drain. And poop with different regularity along with location preferences.
(The second thing being another reason why wastewater analysis is better for drugs/metabolites that excrete through urine)
It may not be a single person, but the individual(s) being infected exhibiting the same movement patterns would certainly be living together in close contact, and the lack of community spread supports the group of patients being very small, possibly just an individual.