That's where the headcanon begins though. Everything meaningful Law did so far in all of his panel time, was when used along with his Ope powers. Outside of his DF powers, he is a rather mediocre swordsman (getting stalled by a clone of Doffy). Using such advanced techniques like fire cutting, usually gets noted specifically and is not just used randomly without further elaboration.
So yes, factually you have to pove that it was a cutting technique of Law used without his DF powers.