They overturned the courts previous decision. Technically it wasn’t a law before, it just was heavily implied (as in Congress specifically left things vague bc they wanted federal agencies to fill in the blanks in accordance to the Chevron doctrine).
Basically, there wasn’t any part that was unconstitutional, they just said the court was overstepping their boundaries when they “created” the Chevron doctrine.
Edit: please read the comment below, it seems like my understanding wasn’t quite right