Supreme court declining to hear the case means the lower court rulings stand, and there is no more courts to appeal to, so they are now The Law Of The Land. There two rulings in question.
Lower courts found with Apple that they can be as restrictive as they want in their store. This is what Epic was appealing under anti-trust.
But those same courts said that Apple couldn't restrict behavior of their apps after downloading. So if Epic wants to sell you a cosmetic in Fortnite, Epic can link you to the Epic store for you to buy it, and make it available on your iPad. That means Apple can't guarantee that they skim off the top of all microtransactions, or prevent apps from saying "hey, this game is cheaper on Steam."
The walled garden has a hole in it.