The case is not a legal issue regarding "soverignty" of the U.S. or the U.N. The case involves contract law and whether or not Mr. Parton was personally obligated to live by the contract he signed - is that contract valid, did he sign it, can he be held liable for abrogating it.
It is not an issue of whether or not the U.S. Congress had a right to the documents. The issue is whether or not Mr. Parton had a right to provide them.
It would be no different if the other party had been Great Britain or Israel or Japan, and Mr. Parton had signed the contract with them. The U.N., in this instance, is no different than a foreign nation.
The entire legal question is "did Mr. Parton break his contract" and "was that contract valid" - not the soverignty of the U.S.
Can't a subpoena override a contract of confidentiality?