This CONCACAF tournament doesn't fall on a FIFA date, and therefore, Fulham would not be obligated to release him. Rubio Rubin, who was also listed on the roster was not released from Utrecht. With Hyndman playing and being on the bench quite often, many didn't think the club would release him.
However, it does make some sense for Fulham to let him go. Assuming the US makes the final, Hyndman will be playing in 7 games over 15 days. That's WAY more work than he would be getting at Fulham. He's also going to be one of the older and most experienced players on the U-20 squad allowing him to build on his leadership skills. In the long run that might just be better for Fulham than having him play a handful of minutes over January. Also, with Alex Kacanicklic and Ryan Tunnicliffe coming back, there is some cover for him in the midfield.
As a fan of the USMNT who came to Fulham because of the Fulhamerica period, I'm always happy to see more Americans at Fulham and relish when they are on the national team. I'm rooting for Hyndman to have a big tournament, helping the USA qualify and then building on that when he comes back to the club.