They aren't, Hara said Ouki was based on Wang Qi/Yi who was a really obscure guy. Seems to me that Ouki was there the whole time based on his comments but it's really wonkyIRL Oukotsu was the general and he was later replaced by Hakuki (Ouki and Oukotsu are the same person IRL which makes it tricky I think) so we don't know how Hara played it but it's likely there was two of them at least. Either Ouki + Oukotsu or Hara simply ignored the replacing and just made it Hakuki/Ouki for the entirety. The wikipage seems to think the latter is the case.
Perhaps, doubt Kyou was there though.Tou was in there also. Was Kyou a Great General then? Because Chouhei was 19 years before the series whilst Kyou died 9 years before. I don't think she was a GG for a decade.
Either way Riboku has a chance here to replicate such feats.
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King Ouken also believes Riboku is number 1 and he definitely knows about Kouen
I doubt it though, Riboku looks to have the more impressive roster and Chouhei was a massive and rather even battle. And funny thing, Renpa was the one with the earthen forts iirc and he held out for two years with them irl.