Yea, but the argument is you have to pay the 30% fee. You can sell keys outside and not pay it. Also you can also sell your game on GOG, but the customer will have to stick with were he bought the game from for the DLC.

https://www.gog.com/en/game/clair_obscur_expedition_33

https://store.steampowered.com/app/1903340/Clair_Obscur_Expedition_33/

In apple's case you can't even buy it outside of apple you have to use apple payment system. Steam doesn't block you from buying the game from other store like GOG, meanwhile apple does.

https://prospect.org/2025/05/02/2025-05-02-apples-monopoly-finally-held-accountable/

The case before the court concerned Apple’s monopoly power over its iOS App Store. Apple has built a tollbooth whereby apps that offer items for purchase must pay a 30 percent tax to Apple. (A few select apps have a smaller 15 percent tax.) Epic Games, the makers of Fortnite, wanted to offer game purchases off the app through a link at a cheaper price point, but Apple barred Fortnite from the App Store for such circumvention, and denied any developer the ability to steer people to off-app purchasing. This discouraged app developers, since they would not be able to load on iPhones and would therefore lose access to a huge number of potential customers. (Google has a similar 30 percent tax for its Android phones.)