It's not PPS, but also not proprietary.If you look closely at the Power Adapter images, here's the specs.
5V - 3A
9V - 3A
(AVS) 9-15V - 2.67A (3A DPS)
(AVS) 15-20V - 2A (3A DPS)
I guess we'll have to see if Apple's AVS and DPS is just Standard USB PPS or if it's another Apple proprietary charging standard.
It's part of USB-PD 3.2.