It is true. Other stores are not your app.

You might have meant something else, but what you wrote is:

> they require apps on the platform to use their payment processing so that they can take that cut.

Which is not true. Both steam apps and in-game content for steam apps can be sold in other stores with steam's blessing, and steam expressly supports developers to bypass their payment processing (by offering steam keys and account integration).

My statement was specifically referring to the app itself. The app is what has to use steam wallet and not something else. Selling a steam key on a website is not selling it in the app. The requirement applies to the app and not the website for the app.

Set app selling aside - I only mentioned that because it's a counterexample to your original post as stated.

What TFA is about is selling in-app content from external stores. Apple used to ban that entirely, and now wants a revenue share for it. Steam has never done either - they explicitly support it (via account integration). The two are not similar.