If TCPP01-M12 integrates ±8 kV ESD protection on VBUS, why is there an external ESD diode on the VBUS line in the reference schematic?

Yes, the TCPP01-M12 integrates ESD protection on V_BUS and CC lines. It is compliant with IEC 61000-4-2 Level 4 with ±8 kV contact discharge.
To protect against much higher power transient voltage surges on V_BUS, we encourage you to add an external Transient Voltage Suppressor Diode (TVS) to protect this V_BUS line against EOS and ESD transients.
In this case, the ESDA25P35-1U1M can withstand peak pulse voltages up to 30 kV (contact discharge) and peak pulse power up to 1400 W.