Shouheikun disagrees, only Moubu could defeat Kanmei according to him. And his opinion >>>>> yours
All this means is that Shou Hei Kun believed it to be true. That's literally it.
Why you continue to take the statement of characters for gospel, I don't know, but it doesn't matter what they
say.
What matters is what Hara
shows you.
Touting the opinions of characters is near enough meaningless in the context you're arguing.
The first, second and last priority of any decent storyteller is telling an interesting story. In service of that, they will often introduce conflicting pieces of evidence, views and perspectives throughout the story. It's called drama.
There is also the question of whether SHK event meant his statement the way you're taking it. Did SHK mean Mou Bu was the only one that could defeat Kan Mei in a duel, or that Mou Bu was the the only one that could take the field and win? The former is likely, the latter not so much based on what we've seen.
In my view, Mou Bu
is underestimated by a large portion of the fandom (as is pretty much any character not defined by their intellect, cruelty and/or "cool factor"), but I'm not buying he was the only man fit for the job. Perhaps he was, but the views of SHK does not set that in stone.