well... it doesn't need to be like that imo. Ousen is a very cautious general that he managed to get info about Yan attacking Zhao when it is more or less completely on the other side of the map. I think it's fair to assume that Ousen has some sort of spy networks to watch over Zhao's own movement (the same way Riboku has some to watch over Qin's). It doesn't need to be "Zhao coming to attack their own great defendable city" but the fact that Riboku army didn't really go into their favorable position to attack Qin.
Unlike normal battle, Qin can't really just abandon Retsubi or their own plan to attack Gyou.
Losing Retsubi = losing supply route = game over
Abort attacking Gyou = Qin attacking less important area of Zhao = Zhao won't fall
Waiting in Retsubi = give plenty of time to Riboku = losing the meaning of the blitz attack in the first place
Riboku can just sit his ass somewhere quite near that has a lot of option to move. With that alone, he will check Qin's army because they can't move well. Going straight to Gyou and Kantan will result on their supply routes being broken, staying in Retsubi itself is risky, while coming to attack Riboku will play directly into what Zhao wants (for the battle to be dragged longer and Zhao capital is saved).
well... they did think about it for a year, but wasn't there some sort of discussion/scene with SBK and the rest about howWhile I assume the supply problem as well, it would be out of character for SHK not to thinks about this over year. They have to take Retsubi, then Gyou and then hold it for three years while fighting for Kantan while keeping the supply chain up all the time. While having an increasing risk of Wei/Chu/both taking their shot.
"we can't really move when we need to have this supply route, let's just abandon the supply route and deal with it"
i think the best they can come up with is that fake supply route to Kokuyou and revolving the supply route around Retsubi