Drawing on multiple research efforts, including studies by the U.S. Centers for Disease Control and Prevention (CDC) and National Institutes of Health (NIH), findings suggest that SARS-CoV-2 was likely circulating in the U.S. well before the first known
https://academic.oup.com/cid/article/72/12/e1004/6012472
I think this is what they’re referencing in the article. I hate it when writers don’t link to sources.