By exclusivity I mean for Apple devices, which is what I cover in the redundancy portion. If you want to own these games on your Mac, the Mac versions are not for sale on Steam. Only the Windows versions. Mac versions are only available on Apple Arcade.This point is not true. For example, the game Sayonara Wild Hearts is on Apple Arcade. But you can also purchase it standalone on Steam, Nintendo Switch, PlayStation 4, and Xbox.
Edit: For your Exit the Gungeon example, in addition to being on Apple Arcade, it is also available to purchase on Steam (for Windows, MacOS and Linux), PlayStation, Switch, Xbox, Google Stadia, and Amazon Luna. I think they covered everything.