Not written by Williams, but arranged by him, and it was the traditional American (?) tune Simple Gifts, which appears in Copland’s work. It was beautiful, but I prefer Copland’s version.
Good catch! I was mixing up my Coplands. I assumed it was arranged by John Williams, but CNN’s reporters clearly don’t know the difference between “arrange” and “compose.” I was listening, rather than watching, so it completely threw me off. Plus, I find it funny since John Williams has a reputation of “borrowing” music from others.