The principle of PD fast charging is to increase the voltage at a certain current to achieve greater charging power. The device will communicate with the charger before PD fast charging, and only after the handshake is successful will it output high voltage for fast charging. If the device does not support it, only 5V voltage will be used for charging. So you won't have any problems charging the iPad.
Not only the PD fast charge protocol, Qualcomm's QC fast charge protocol, MTK's PE fast charge protocol are all the same, they will shake hands with the device before entering the fast charge, and only after the handshake operation is completed will the voltage be increased and enter the fast charge mode.