I noticed that there is no mention of this in the RRG:

All retreating units must move to the same area, and it must be an area that at least one unit in the combat moved from .

It mention it for units moved via another mean than Advance Order, but for units moved via the Advance Order, it is not said anywhere but in the Learn To Play.

I'm guessing its just an oversight

RRG is supposed to integrate what explained in the LtP, and in case there's a contradiction, the Ref has priority. This doesn't mean what's in the LtP is to be forgotten

You're absolutely right and in this case it wasn't forgotten as we can see. But the RRG should include all the rules and this one seems to have been left out.

This was pointed out on BGG as well, so I tossed a question to the rules folks at FFG.