self reply: issue solved
- Upgraded the Flirc SE firmware to the latest available for gen1: v39
- Re-recorded the Wake key (Harmony is programmed to send "FLIRC > AspectRatio" as the power-on command.
made sure that "sleep_detect" is activated.
It just worked; turning off other Activities like Netflix will no longer cause the HTPC/Flirc to wake up. Before, I had v37 and it seems sleep_detect wasn't working.