Technically, OEM (original equipment mfr) is whoever makes the part(s) for the final product, which means Nissan is not always the oem on specific part within the vehicle. Most auto mfrs source parts from all sorts of suppliers, including this one.
The term is thrown around for anything you could get at nissan dealer as a replacement, for the most part. Probably not right, but accepted....it is like calling an exhaust tube a pipe, an engine a motor, or a screw a bolt (which is only true if it has a nut).