Serial refers to the electrical signal levels and the timing. It is common to put serial interfaces on a "DB-9" connector, so people often equate the "DB-9" with serial. In the case you are referring to, it is a serial interface, but does not have a "DB-9" connector on it.
You just need to make the connections as detailed by the backpack manufacturer. You can use an IDC connector on a ribbon cable with a 2x5 0.1" center header. Solder the wires directly to the backpack, whatever you would like.