Devs could, for example, charge a higher price if you buy it via the AppStore and a lower price if you complete the purchase on their website. It potentially removes a significant portion of Apple's cut for IAP. Apple also has to figure our how to collect the "commission", which could lead to devs not working with Apple in the future. Apple will have a hard time appealing because the appeals court could rule against them on the counts they already won on. That's why it's not seen as a huge win for Apple.
Epic has to pay a meaningless amount of money (a few million dollars they'll make back up in a few days) and is barred from a platform they didn't care about in the first place. Non-US regulators are also looking into Apple's handling of the AppStore, which means Apple could still be forced to make changes in line with what Epic was arguing. This is why it isn't a huge loss for Epic.