The China travel ban might have helped, I don't think there is enough evidence one way or another to know for sure.
What we do know is that Europe has tested a higher percentage of it's people, of course that is going to identify more with the virus, that is it's purpose after all. We do know that the USA and other countries were offered tests from the WHO. Other countries took advantage of that opportunity and have been testing at a much higher rate. The United States is under reporting cases because we haven't bothered to find them. Now, finally while there is talk of what is going to be done in this regard it still hasn't happened yet. We didn't take the time when we had it to ramp up. This will cost lives, this will continue to be a hindrance on the economy until we get it under control.
The downside to travel bans is evidenced here. We look at countries in Europe, they have mixed results. Italy certainly dropped the ball but other countries which are identifying more people per million than we have due to testing gives the impression that they are worse off and therefore a travel ban is necessary. So, if a country really depended on the United States travel for it's economy then that country would have an incentive to test less, identify less sick people. At this point though, no matter what side you are on with travel bans it really doesn't matter, it's here, it's spreading.
So, even if the China travel ban helped, we didn't do much of anything else to ramp up for the eventual arrival of the pandemic. We didn't do anything to build up infrastructure within our borders to conduct testing, didn't take advantage of the WHO offer to get testing until we had our own, didn't do anything to alleviate our very possible overrun of our healthcare system. Didn't do much to communicate and prepare our populace not to panic. Instead we had contradictory information from the White House and our healthcare institutions (public and private). It exasperated a problem this country has been having for the last three years, a lack of trust in our national leadership.
We're going to and have been learning a lot about ourselves in this.