the 1st gen 12.9 does support USB C power delivery, but not at any voltage. The mac charger will work but some other power delivery chargers do not work as they do not support the same voltages (contrary to what people may believe, power delivery can be implemented in different ways, some devices support all possible voltages, 9v, 12v, 15v, 19v, 20v, others only some of them, and if they don't match they revert to 5v)