That unit has an ARC HDMI port, but that unit was released in 2008 and HDMI ARC was released in 2009. I suspect that Onkyo jumped the gun and that the HDMI ARC spec on that unit is not correct.
In any event ARC was very hit and miss, with a lot of problems. It was not until eARC that it became truly reliable.
I would regard that unit as not having ARC.