That article DOES say they no longer exist on Saipan. There are TONS of ganebirds, because cock fighting is really big there, but they were orginally imported, mostly from the Phillippines.
SO basically, some GI in WW2 brought back the "mutt" gamebirds they were using for cock fighting on Saipan and decided to market them as a breed. Enterprising guy!
There is a native jungle fowl, but is is more like a marsh hen or duck than a game cock! It's called the Marianas Mallard (except for the webbed feet, does not resemble Mallard ducks)