For me the big reason is that the manga has stressed that Ousen has his weaknesses for years now and Ouki really doesn’t. He is like Tou, a balanced GG with zero weaknesses. Ouki can body bag you martially, or Ouki can body bag you with tactics and strategy.
Ousen is pretty good on the plains, but like Riboku his true strength lies in the way he can earn strategic victories outside of straightforward battle. Which means Ousen’s true ability to shine (Gyou) is situation specific while that is not the case with Ouki.
Yes I don’t think Ouki was capable of Gyou, but I also don’t think Ousen is capable of a lot of shit Ouki was. And recall before Coalition, Ouki was the only Qin general Riboku thought it was necessary to eliminate. Overall I do think Ouki was probably the strongest Qin general at the beginning of the manga even if Ousen outclassed him strategically and Moubu outclassed him martially. Ouki was versatile on a level that Moubu and Ousen are not.
As for why I ranked Moubu > Ouki, it’s mainly just from how severely Shouheikun has hyped Moubu since coalition. I put a lot of stock into what credible sources think in the manga since they probably know much more than we do about stuff like this.