Seems like a risky move, also why not just head for the Shukai plains? They would have defeated the Ousen army if they did.
And then there would’ve always been the possibility (probability actually) that the Ousen army would’ve been able to retreat back to Qin (alongside the Kanki and Yotanwa armies)
Recall how dangerous Riboku thinks Qin is because of Sei’s ideology. Zhao wasn’t just trying to win this battle, they were trying to eliminate a 200,000 man Qin Army completely. Hence why the Retsubi fail safe was being used:
1) to stall the Qin Army out of food and prevent them from reinforcing themselves
2) to prevent Qin from leaving Zhao easily.
Shunsuiju reinforcing Riboku would mean that Ousen could just turn tail, run away, and the Qin could retry a similar invasion in a year or two because the Zhao didn’t take the time to eliminate them then and there. And Zhao would face the exact same issues as they faced during the Gyou invasion.
Tou arrived at Retsubi with an army of 40K. Akou also came with reinforcements but I doubt their numbers would exceed the 100K. Especially when you consider that Retsubi was already fortified with 50K soldiers before Ryouyou was captured. Shunsuji's army in combination with Kouchou's should have been twice as big as Tou's and Akou's combined.
Kochou and Shunsuiju didn’t retreat because of the size of the Qin Army and the generals commanding them, they retreated because there was no longer strategic viability in defending Retsubi since the Qin had lost Gyou. The whole point of Retsubi was to lock the Qin within Zhao with no strongholds for them to retreat to, but they didn’t have that option anymore since the Qin had taken Gyou.
“Well why didn’t they just hold Retsubi to prevent Qin from reinforcing Gyou?”
Iirc, Kochou took his 100K elites to Retsubi, right? Ousen now has a fully fed army with an open path to Kantan. A fully fed army that just defeated a Shukai Plains army effectively commanded by Two Great Heavens.
You could recall Kochou’s army to Kantan and leave Shunsuiju to protect Retsubi, but in that case, Tou and Ousen will just sandwich Retsubi and brutalize Shunsuiju and his 100K army there. Kochou could attack Ousen from behind, but this would leave him vulnerable to counter attacks from Yotanwa and Kanki, and would also leave Kantan unprotected.
The best option for Zhao was to consolidate all of their forces at Kantan and double down defensively, which might I add has worked extremely effectively as the combined forces of Yotanwa, Ousen, and Kanki as well as Mouten, Ouhon, and Shin have made little to no progress since the Zhao took up this strategy.