Earlier quoted context omitted.
And what evidence suggests that travel bans actually do anything to "slow the spread"?
Math? Any particular transmission event is chance of someone being infected X probability of transmission. Reduce the first term and the total number of infected individuals at a given time T is reduced (until any exponential growth swamps the terms). Point of saying "it's impossible to prevent" is that it's not possible to reduce the first term to 0.
but isn't that almost immediately the case? Let's say you have only 50 people in the States who have the new variant before you notice it, with an r naught of 2-6 that most of these strains seem to have after a few days you're probably already way beyond the few falsely negatively tested people who come in by plane. (the US still requires tests right?)