As I understand it, 18F started off as part of USDS. But the Obama administration wanted the program to outlive his tenure as POTUS, so they tucked 18F into the GSA.
That's not correct. 18F predates USDS by about a year, and we are (and always have been) independent of each other, though there is a lot of cross-pollination in our work.
Was talking to a Presidential Innovation Fellow, and probably just misunderstood what they were saying, or perhaps the Fellow had incorrect information.