The march 2019 finding is almost certainly a false positive. This is the more interesting and potentially more robust finding from the paper:

"Unexpectedly, analysis of archival samples revealed the increasing occurrence of SARS-CoV-2 genomes in samples from January 15 to March 4, 2020 (Figure 1, panels D and E). Of note, SARS-CoV-2 was detected in sewage 41 days (January 15) before the declaration of the first COVID-19 case (February 25), clearly evidencing the validity of wastewater surveillance to anticipate cases in the population."

This is unfortunately likely to be overshadowed by the march 2019 thing, and I'm surprised that the authors felt comfortable drawing an actual conclusion based off of the 2019 finding and presenting it as "indicating" that widespread circulation was occurring nearly a year before the pandemic hit worldwide circulation. It's not responsible science and might draw doubts on the validity of the other, more interesting conclusion with more evidence to back it up, and rightfully so.