It's not a devil fruit ability because the gorosei don't have devil fruits. Nusjuro is just a master swordsman at the pinnacle of swordsmanship
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It's not directly his devil fruit, no. His devil fruit literally just gives him the power to exist as a soul bound to his body after death. He can use his sword techniques as a consequence of being dead, but they're not "devil fruit powers".
Anyone with a connection to the underworld can use soul sword techniques like this. Eating brooks devil fruit doesn't automatically give you these powers.