From what was said, because he didn’t have a set school or style of fighting, plus his sword could transform. His named attacks didn’t really have anything to do with his sword, maybe that affected things.
Kaku used Yontoryuu, so even though that had non-swordsman attacks the style/school was enough.
Cabaji had his Acrobatics move which apparently counted. And no, I don’t think that’s consistent, but chances are Oda doesn’t even remember the Cabaji fight, so eh.
Fujitora’s fighting style is called Grav-blade so it should count.
Law actually talks about swordsman spirit, has a cursed blade which we found out this arc needs to be mastered by a swordsman to be used and would be next to useless in a fight without it, so he should count as well.
Big Mom knows how to use a sword, that’s for certain, but I think she would fall under the King category since she also uses her homies and punches too much.
But like I said, it’s not consistent. Zoro’s done things that shouldn’t count as swordsmanship. He also said in the chapter that was brought up that if he would do anything he could to win the fight as well. So is he disqualified then as well?