: My best guess is that Astin was referring to the Dead Marshes which Sam and Frodo pass through on the way to Mordor. With all the mixing-up of the times and locations in the story due to the shooting schedule, maybe it is possible that Sean got mixed up and when he said "Swamps of Mordor" he meant "Swamps NEAR Mordor"? Or maybe he isn't that familiar with the book and thought that the dead marshes are actually IN Mordor?

I agree that the reference is probably to the Dead Marshes. Although anyone can look at map and see that the Marshes lie outside the mountains that demark Mordor, designation "Swamps of Mordor" is not necessarily incorrect. Geo-politically, the Marshes are under Mordor's control at the time of LOTR, and are therefore a territory of Mordor.

After all, Minas Morgul is outside of the mountains, too, but I don't think that anybody would object if I called it "a Citadel of Mordor". Would they?