Here's an example, Samurai Jack's Sword is magic, but in the hands of evil it cannot harm an innocent person. Superman is an innocent person. So the magic sword would not harm him. See how that works? Magic does what it's designed to do. It doesn't automatically weaken superman just because it's magic.
Here's a another example. Just because the killing curse (from harry potter) can kill does not mean all magic is that effective. The whole point of the curse is to kill. It's an absolute spell with an abstract effect similar to the death note. However simply using magic to say....start a fire would not burn superman. Because it just burns like regular fire regardless if it's magic. Unless the magic fire is specifically enchanted to literally burn "anything" instead of just burning like a normal fire would. This logic applies just as much to someone like the Hulk as it does Superman.
Magic is more nuanced then some might have you believe. After all, The hulk can survive a blow from Thor's Hammer, and he isn't anymore immune to magic then superman is. The hammer is enchanted to hit you very hard. Not "one shot kill you". The hammer would likely kill a normal person because well...we mortals are fragile to normal hammers as it is.
Magic is not a special "only effects superman" type deal. It effects superman the same way it effects everyone else. It will effect batman the same, it will effect the flash, the martian manhunter, you get the point. the point is it's magic and magic basically messes up anyone unless they themselves are specifically protected against magic like the phantom Stranger or something like that. Even Reed is specifically seen telling Ben Grimm that he is vulnerable to magic, and hellfire is shown burning Johnny due to the magical nature of the flame. Superman may be vulnerable to magic but that doesn't mean magic is Invulnerable to Superman. Some types of magics have limits just like everything else.