Thing is Jack's weapon literally kills evil. When Aku gets attack by Jack's sword he can only regen some of his body at a time but it eats away evil. If you are pure of heart the blade can not harm you for tbf it was a kids show xD
Unless we're assuming Jack's sword would null Muzan's regen completely (not sure if there's any basis for that?) he gets stomped hard. Last chapter shows cutting damage does shit to Muzan because he heals so fast everything passes through him. He's low on DC feats but his whips should be enough.