I believe they are referring to the Turkistan Islamic Party, who aim to establish an Islamic state in East Turkestan i.e. Xinjiang. Some of their fighters went to participate in the Syrian civil war, where ISIS losing territory might prompt them to move back east. While this is the result of the US fighting ISIS, making it sound as if the US was actively helping ISIS fighters relocate is disingenuous.

https://en.wikipedia.org/wiki/Turkistan_Islamic_Party_in_Syr...