1) the Logitech Harmony sends a long button press to the Mac Mini to put it to sleep. There is no other way to do this over IR.

2) Remote Buddy allows its users to map short and long button presses individually - including the button whose long button press the Harmony uses.

3) By default, said long button press is not *globally* mapped to the sleep action, because most people prefer to use it to access additional functionality.

4) If you want to put your Mac Mini to sleep in reaction to said long button press (which is the only mechanism available to the Harmony Remote), follow the instructions I posted as part of my previous answer to hardwire said long button press to the sleep action.

Best regards, 
Felix Schwarz