Well, I said "Ousen" plot armor, not Riboku.
We have seen Kanki sieging Gyou, dude and his goons were right outside the walls for the all time, probably just outside arrow range.
Even Kanki's tents was right there.
https://mangaseeonline.us/read-online/Kingdom-chapter-589-page-11.html
https://mangaseeonline.us/read-online/Kingdom-chapter-589-page-12.html
Meanwhile Kaine is one mile away, and the Zhao army is God knows where lol
http://sensescans.com/reader/read/kingdom/en/59/638/page/17
So yeah this to me is not Riboku getting outplayed or Ousen being smart, it's Hara writing himself into a corner and then coming out of it badly.
But I don't care much tbh, I'm more interested to see where we go from now, because this campaign is not over. Gyou and Ryouyou right now are basically isolated Qin's castle inside Zhao's heartland. So in theory Qin would need to destroy the armies surrounding the cities and capture Retsubi for good while keeping Gyou and Ryouyou which seems like a fairly long thing to do unless pedoking comes in and stupidly orders all the troop to go back to Kantan to protect his ass.