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The Xiaomi Mi 6 supports both USB Power Delivery and Qualcomm Quick Charge for fast charging over USB Type-C (which should be based on Qualcomm Quick Charge 4+ technology) and doesn't use any proprietary quick charging schemes. This means it should be able to work with other products even from Apple that support USB Power Delivery such as the Apple 29W USB-C Power Adapter.

Using the Granite River Labs GRL-USB-PD-A1 test software, we can see how the Mi 6 phone charges with the Apple 29W USB-C Power Adapter  using USB Power Delivery from a 2% battery level on the phone. The Apple charger first repeatedly offers 12 watts (5 volts @ 2.4 amps) and 29.6  watts (14.8 volts @ 2 amps) to the phone when connected directly over a USB Type-C to Type-C cable.

The Mi 6 then responds with an initial request of 12 watts (5 volts @ 2.4 amps) and is accepted by the charger. This is later followed by a series of Vendor Defined transactions between the devices.

The power graph further verifies that the Apple 29W USB-C Power Adapter actually delivers about 11.5 watts (4.8 volts @ 2.4 amps) to the Mi 6 at power on starting from 2% battery. Since we only observe the phone getting close to 12 watts, it can still charge properly with the Apple charger over USB Power Delivery.

Compared to the USB Power Delivery charging seen with the Apple 87W USB-C Power Adapter in this review, this charger provides more power to the Mi 6 than the Apple 29W USB-C Power Adapter even though these measurements were taken around the same time and at the same battery charge level.