The approach is different. Has motors have interface to accept commands and ROS can send commands. But the control loop is still closed inside ROS.
When I say control loop I means the one that is looking at error in the 6 DOF arm’s end position. ROS looks at this error then sends message to each motor controller. Machine kit does this calculation inside a lop that runs 1000 or more times per second.
A good division of labor might to have ROS send a message like (go to x,y,z with orientation a,b,c) and ROS might update this 10 or 20 times per second. Then MK was direct control of all 6 motors in the arm and update the motor control signals 1000 ties per second. MK is not just a dumb motor controller. It can compute the kinematics and plan a way to correct error in the trajectory
Again the defiance between using 6 motor controls and ROS vs. using MK is that MK can do kinematics.