Putting the words of Koushirou under perspective - he's a swordsman in East Blue from Wano heritage who has never seen a CoC user in his lifetime. Even Wano citizens haven't, outside of Oden & (very possibly) Ryuma. They don't really know what Conquerors haki is.
For example, Ashura Doji sees Luffy unconsciously knock out fodders after Kaido one-shot him and calls that "Oden-sama's trick".
Kiku sees Luffy knock out Beast Pirate goons using conquerors as they go into Bakura town to save Otama in act 1 and is stunned looking at that.
Kozuki Oden, a CoC using swordsman from the land of Wano where flowing CoA/Ryou is commonplace, has no idea what he was hit with after Roger sends him flying with one Conquerors attack. He sees Newgate & Roger clash without touching at all and is stunned at what he sees, and has no idea what they are using.
Later he comes back out of his journey being much stronger than what he was before by learning what he couldn't in Wano - the ability to infuse his attacks with Conquerors -> scarring Kaido feat. And even he couldn't forge a Kokuto in his lifetime, which Ryuma & (most likely) Mihawk accomplished.
It's pretty clear that infusing one's blade with conquerors haki is a necessity for anyone who wants to reach the pinnacle of swordsmanship. Forging a kokuto as a feat comes even after that most likely given what the sample space we have indicates (Oden, Zoro). And in that way, Cinera is right.
Koushirou was only talking from what he & his ancestors know, they don't even know the existence of a stronger form of Haki in some select, gifted individuals so that's what his knowledge is limited by.