I thought the mobile key was to use your phone as the physical item you needed instead of the key... And that you needed digital key prep option on the car to do it. With that you can also send the key to someone else and revoke them as well (eg send it to a friend so they could get something out of the car and then stop them using it after they had done it)
I think from the we connect app you can only see if the car is locked or unlocked, not actually lock it.