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.
