Well the book only came out a few months ago so it can't be too old. I don't think they would change a detail like that in the last month or so of production anyway.
My theory was that the Mongo river was the same one that MO started at because I can't imagine the gabbits travelling across a continent to breed (you can see my justification in this thread
http://www.oddworldforums.net/showthread.php?t=11103) but it was pointed out that SW occurs in Western Mudos - but I don't know if that has been confirmed recently or not.