Okay so I'll start over again.
---- HOW do they know --- IN NEW YORK --- that they didn't get it from cross-country travel out of Washington and California (INTO New York), rather than from intercontinental travel from Europe?
Why are you asking me and why point the finger at Washington and California?
During containment, there was very little of people who had COVID-19 go from California to New York, if any. Even if there were, are you going to believe a few of those people would cause what happened in NY and NJ? Why don't you believe the article that the strain in NY and NJ is an European version. That fits the other countries in Europe being hit the hardest including the UK. It means we start tracking for different COVID-19 strains.
Because Washington and California were where it first appeared. It stands to reason that cross-country travel to New York, which was and is both unrestricted and busy, should have brought some silent carriers. It seems more likely that wherever your first cases show up, they're going to spread where they travel. Certainly NYC is a major destination for travel from Europe but it's also a major destination for travel from the West Coast.
What you posted earlier seems to want to ignore the latter and single out Europe. That's not logical.