High output voltage or sink?
Let's check the datasheet:


  High-level Output VOH on HVOUT min. 78V for IOUT = –25 mA, VPP = 80V


So, if the output is high, and your load uses Iout=25mA, the output voltage will not be exactly VPP=80V but around 2 volts less. So it is suitable for driving a long string of LEDs at 20mA.


  Low-level Output VOL on HVOUT max 3V for IOUT = 1 mA


When the output is low, the driver's strength is much lower. It is only able to sink 1mA with a voltage drop of 3V. It can't drive LEDs in this mode.

In other words, the output can source 25mA and sink 1mA current, it is asymmetrical, so you should put the LEDs between output and ground.