I mean even after Moubu "got started", Man'U seemingly had an upperhand.
look how casual he was at the end of their duel
man was smiling while blood was coming out of Moubu's mouth.
even outside of the duel, Man'U had pretty fantastic hype. The strongest state Chu "gave up" against Man'U after he defeated a significant number of Chu generals. Beating tons of Rinbukun tier guys, and repelling Great General Kanmei's army.
I don't know how strong Man'U is or if he's even supposed to be Chu's strongest, but he's absolutely up there.
You recall when Man'U claimed he was weaker than Kanmei and then it was questioned by soldiers? This is an interpretation I had in the past: Moubu never truly was stronger than Kanmei and only won because of the "burden" and his greater weight. So it's possibly Man'U could be weaker than Kanmei and also onpar/stronger than Moubu.