Actually, Apple blocked the Steam remote play app because it just acted as a remote desktop for Steam's Big Picture mode, which also allowed using the store for making purchases. This was a thorn in Apple's eyes so Valve had to ensure the shop was inaccessible when opening Big Picture mode through the iOS / iPadOS app.

Source:

- https://www.theverge.com/2018/5/24/17392470/apple-rejects-va...

- https://www.theverge.com/2018/6/5/17430532/apple-app-store-r...