The ruling was basically that the tariffs themselves were unconstitutional. Specifically, the court found trump's tariffs so arbitrary that congress is forbidden from delegating the power to impose such tariffs to him. In order to not void the laws congress passes, the court can reinterpret the wording of the law so that it stops being unconstitutional. This leads to some explanation that the particular law he invoked gave him too narrow a power, but any law you can mention, no matter how broad it seems, would be interpreted by the court as too narrow (or as unconstitutional). Trump's argument in this case was basically, well I can declare an emergency any time I like, and I can claim that whatever I'm doing is fixing the emergency, so I can do whatever I want. Even though they might otherwise have given him that broad discretion, in order to maintain separation of powers they had to call bullshit on that reasoning.