Kinemon was ready to slice Jack up. The Dukes only stopped him because they had a score to settle with him.
We also know that even Kanjuro, once getting serious, was quite a threat. Able to destroy a fleet with ink shaped into needles raining down, creating flaming Yokai that burn everything down and creating Oden clones that stab Ashura to death. We know Ashura could go toe to toe with Jack after blitzing him, Denjuro could easily stall Nitoryu Zoro, the Dukes even in base come close to a commander, let alone in Sulong form.
But somehow their leader is the only one that is supposed to be fodder. People just have cognitive dissonance. It is as you said; the participants of Punk Hazard (Vergo, Smoker, Law) were just that strong, that Kinemon looked weak. It's not that Kinemon was actually weak.