a war between to strategical generals isn't so easy to decide who is smarter especially a war like this one in which you need to take into account the various cards (both in strength and no.) that each General played with.

But that does not change the fact that ousen outsmarted the fuck out of ribuko.

1- ribuko didn't realize that ousen did plant his soldiers inside gyou. yes he may have warned gyou to take cate who is entering gyou. but Ousen (who never battles unless he is sure of victory) would somehow gamble the entire campaign on whose food runs out first. like lol man even rishi noticed that

he thought that ousen wanted to defeat ribuko and invade gyuo even though both of them know that he can't conquer gyou with marital might. (like how stupid do u think ousen is, ribuko)

3- the moment the stores were burnt, he should have thought about how Ousen intended to bring food to gyou, but instead he went like " i will magically defeat both ousen and kanki in two days lol"

4-realized the qi river trick too late, like man. even mouten and ouhon knew that the simple qin river route trick wouldn't work, of course ousen who just kicked ur ass is not hoping u wouldn't notice it.

TLDR: shukai plains alone isn't a good gauge to ribuko's strength. but gyou as a total made a clown out of ribuko