Checked again on a more recent KWin and software brightness is now working for me. Confirmed by making BacklightBrightness inoperable, restarting powerdevil, changing brightness slider and checking that /sys/class/backlight/intel_backlight/brightness stays the same while I still see brightness adjustments.
So the fix will be in KWin. Let's leave this thread open until there's an issue or MR for removing brightness controls from outputs with previous brightness device assigned?