At this point, there really is good DNA evidence that this is untrue. The strains that circulated in Wuhan did arrive in Seattle and California by January-February, but although they spread a little there weren't explosions. The infections in the US are dominated by one strain that entered through New York - after that same strain blew up in Italy. There's a trackable mutation in the spike proteins that is present in the NYC arrivals and absent from the California/Washington arrivals, and right now the whole country is dominated by the Italian version. It's even more prevalent in California and Washington today than the original strains were.
Something like 80% of the infections come from 20% of the infected individuals. If you only have a handful of people sick and they stay home, you don't have to have a whole office get sick, so it doesn't have to explode from a single case. Even if there were early arrivals in Washington and California, they didn't spread widely or trigger substantial numbers of cases. The New York cases were the ones that exploded and spread, and those cases were imported from Italy in February.
Whether that means the Italian strain is actually more contagious than the original Chinese strains - one paper has proposed that, but that is still being worked on.