Your picture shows head unit model "GX-204JE". That very well could have been the one that was fitted by the factory (especially since your armrest console has the blank panel where the OEM aux jack would go).
The rear of your HU has a proprietary 8-pin socket for the aux input. Your car has an existing harness connected there, with wires for audio steering wheel controls (if present on your wheel) and a signal related to illumination dimming.
Here in North America, people have been retrofitting 2007-2009 HU models (which have aux capability) into 2005-2006 Legacys/Outbacks (which don't have aux). That is where people need my i85 transplant harness. However, Legacys/Libertys/Outbacks built in Japan use a different physical connector.
For a time, Subaru offered a kit (P/N H6210AG000) to add an OEM aux jack to Japan-built cars like yours. It consisted of a "pod" ("aux terminal") that snaps in place of that blank panel, and a wire harness which connected from that to the rear of the HU.
The harness in that kit is roughly similar to the one you found from Beat-Sonic, except instead of a pair of female RCA jacks, the OEM harness has a long leg that reaches to the armrest console and has another proprietary 6-pin plug, to fit the socket on the bottom of the OEM aux pod.
See this thread:
DIY: AUX-IN after mount + Center console removal -Legacy 2008
(be sure to see my reply in post #17 there).
I think someone posted that Subaru no longer offers the OEM kit, though some vendors appear to still have them available.
The aux "pod" is still available separately here in the US, but a few years back the price skyrocketed from ~$15 to around $52 from the online discounters (MSRP $70
Note that North American models arrange the aux jack slightly differently:
But I think the same aux pod fits cars built in Japan.
I can take a similar aux harness intended for Imprezas and Foresters (and still available here) and modify it to work with the Legacy/Outback aux pod. That involves re-pinning the 6-pin plug at the aux jack end, and adding the extra 8-pin socket at the HU end to pass through the signals from the car's existing harness.
In your case (since you've already ordered the Beat-Sonic kit), you might want to add a panel-mount aux jack such as this:
(click to see larger image)
...and mount that on your car's blank panel.
There are similar kits available on eBay, including variations that include a USB socket (which you might want for your future after-market HU).