In fact, this is Ukraine. Most of the Danube delta is in Romania, except for the northern part of the Chilia (Kilia) branch which is in Ukraine, and which is what the above picture shows. You can compare with the image in this link
to convince yourselves. So the author is not completely wrong, but the text should read "The Chilia branch of the Danube cuts a jade path across part of the Danube Biosphere Reserve...".