The newcomers, who became known as Sumerians, spoke an agglutinative language unrelated apparently to any other known language.

Just to be pedantic, they are no records of any languages related to Sumerian. But since Sumerian is Earth's oldest recorded language (may have tied with Chinese, but all the evidence is not in yet), we don't really know if Sumerian was an isolate or not. We don't know what the Ubaidians called themselves (their name comes from a placename) or what language they spoke.