The Harmony only sends Apple IR codes, whereas it's Remote Buddy's job to convert these into actions.

By default, if no controllable application is active (and/or running in the background), Remote Buddy will use the Standard Behaviour and its mapping table. In that mapping table, the Sleep action is assigned to the long press of the Play/Pause and Select buttons.

If you don't use Remote Buddy, OS X will also react to such a long press on the Play/Pause or Select buttons by putting the Mac to sleep.

So what the Harmony is likely doing is to send a long Play/Pause or Select button press when you press the power button on the remote, which - provided no controllable app is active (and/or in the background) - triggers the action at Prefs > Mapping > Standard Behaviour > Select (long).

You can change that action there.

I'm not aware of any way to assign an alternative Apple IR code or command to the Harmony power button on a long press, though.

Best regards, 
Felix Schwarz