The diagram in the user manual indicates a 4.7kOhm on-board resistor to pull the line up to 5V.
In open collector mode, the line is either driven to GND, or it is left open and is then pulled up to 5V via the resistor. If the load is not of sufficiently high impedance (relative to the pull up resistor), then the voltage across the load will be too low. By adding the external 750 Ohm pull-up, you allow the 6009 to drive a load with a lower impedance.
Best Regards,
John Passiak