Wasn't the walls created so that they don't die from Wano submerging?
Oden said that Wano country closed to protect it from an outside power, though we don´t know for a fact that it was the government, but there´s not many other options. The sinking was done by the government anyway though, So Wano is still the country on the defensive here.
Considering the Navy was wary of the Samurai, it is not that far-fetched to say that Ryuma fought the actual WG regardless of Imu being present or not.
The Navy is wary of the Samurai as an unknown military force, not out of some historical loss. If that were true, we would have to assume the navy remembers fighting the Samurai, but Wano country doesn´t for some reason, since Oden was shocked when WB told him the navy was even a thing.
