Please clarify Bank Shot Game manual

Robot – An operator controlled vehicle designed and built by a VEX IQ Challenge team to perform specific tasks on the field. The robot may be constructed using only the VEX IQ platform parts and mechanical/structural components from the VEX Robotics by HEXBUG product line. No other parts will be allowed on the Robot. Prior to participating in matches, each Robot will be required to pass an inspection. Additional inspections may be required at the discretion of event personnel.

What are VEX IQ platform parts? Can I use a VEX differential (not made by HEXBUG)?

The VEX IQ platform consists of the various parts specified on Robot rule <R6> has more details.

Yes, you can use the VEX IQ Differential, as it is a VEX IQ robot component.