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The Anker PowerPort+ 5 USB-C with USB Power Delivery can charge the Apple iPad Pro over USB Power Delivery but not as fast as using the the Apple 29W USB-C Power Adapter.

Based on the protocol trace taken with software from Granite River Labs, the Anker charger supports 15 watts (5 volts @ 3 amps), 27 watts (9 volts @ 3 amps), and 45 watts (15 volts @ 3 amps) power profiles.  However, the Anker charger is issuing source capabilities that advertise different PDO's, some source capability advertises 3 PDO's (5, 9, 15 V @ 3 A) while other source capability advertises 4 PDO's (5, 9, 15 V @ 3A and 20 V @ 2.25).  Issuing source capabilities that advertise different PDO's violates USB Power Delivery Compliance.

The Apple iPad Pro needs 14.8 volts for charging quickly over USB Power Delivery but for some reason the iPad Pro only requests 12 watts (5 volts @ 2.4 amps) of charging from the Anker PowerPort+ 5 USB-C with USB Power Delivery.

Note when the Apple iPad Pro is already well charged, it doesn't pull a lot of current from the Anker PowerPort+ 5 USB-C with USB Power Delivery.