People are overthinking this matter.
Law has not learned the ability how to cut fire, lightning or the fusion of both, unless it is confirmed otherwise. His room cuts wont affect intangible things like fire and lightning, they would instantly put themselves together. Radio knife could be considered if the target was tangible but he didnt name it nor is the cut the same to previous Radio knife even tho it necessarily doesnt have to be...
That leaves only one option, Misery is a tangible construct such as rocks. Law doesnt need special techniques or DF ability to cut rocks either. We even saw proof that Misery is tangible when it slammed its hand into the ground. Basically, cutting Misery is like cutting rocks, the most basic feat. People are just confused because they think shes intangible.
With so many fire users out there, Law will have plenty of chances to prove whether he can cut intangible things or not.