You should read the twitter string - it's by a researcher in Seattle and the project that went back and looked at all flu test samples from January and February (when they weren't testing for COVID) to see how many were COVID, and none in January and a handful starting in late February.
"We tested 3600 samples collected in Jan 2020 for COVID-19 status and found zero positives. We tested 3308 samples collected in Feb 2020 and found a first positive on Feb 21 with a total of 10 samples testing positive in Feb."
"As you may know, seasonal coronaviruses are responsible for ~30% of common colds and are easily distinguished from
#SARSCoV2 (the virus responsible for COVID-19) in molecular assays. There is no chance of confusion between these in our assay."
"For (2), the genetic relationships among sequenced viruses reveals their transmission history"
"There have been hundreds of viruses sequenced from infections in the USA. We can use these sequences to date the arrival of the epidemic. Doing so, we see that there were multiple introductions driving the US epidemic and the earliest was in Jan."
"There were multiple paths the virus took to reach the US. There was a direct introduction from China that occurred in late Jan and there were multiple introductions from the European epidemic that occurred during the course of Feb. "
"If we restrict to viruses sampled in California (highlighted here as larger yellow dots) we see that they fall in with the rest of the US epidemic. There is no chance SARS-CoV-2 was circulating in California in fall 2019. Circulation in CA started in Jan or Feb 2020."