Korean imports into the United States went up somewhat because the U.S. economy was recovering, but what's interesting to note is that the two areas where they grew the most were autos and ICT goods. The auto tariff hasn't come down yet under KORUS, and ICT was duty-free to begin with. So the increase in Korean exports to the United States are not relevant to KORUS.
The other point that's often forgotten with those statistics is that the U.S. runs a very large services surplus with Korea, to the tune of about $10 billion. Those figures are only on the goods side. If you bring in the services side of the equation, the picture looks very different.