First, the one you brought is not a proof but a suggestion.
Second, a non-CoC fighter can be just as dangerous as a CoC fighter while relying on different skills.
It's not a prerogative, people like Law are contenders to people like Zoro without being a CoC user.
I don't think Shiryu is a CoC user. In that case, he can make a difficult fight for Zoro (if he really is his opponent) even without that..
Awakened Suke-Suke + Developed & Lethal Swordsmanship makes for a troublesome opponent unless you are really effective with CoO.