according documentation, arduino mega use for control gripper, no stepper motor. the arduino is optional and not important if you use pneumatic gripper.

if it is optional, how do you control the motors ?

the stepper motor controlled by Teensy 3.5, not using arduino mega.

in assembly process, the arduino mega connected to 8 channel relay and standby, you can use it to control more device later, such as motor, buttons, led and etc. it no use for arm movement, calibration