That is NOT what they have ruled. They have ruled that there is a presumptive immunity. That means it can be challenged upon judicial review.

The President enjoys no immunity for his unofficial acts, and not everything the President does is official. The President is not above the law. But under our system of separated powers, the President may not be prosecuted for exercising his core constitutional powers, and he is entitled to at least presumptive immunity from prosecution for his official acts. That immunity applies equally to all occupants of the Oval Office.

Roberts then goes on to explain how all but one of the charges need to go back to the courts to determine if he was acting in an official capacity.