I believe the issue is on access to the target operating system (iOS) rather than storefronts.
Steam does not sell iOS software. They sell games for Mac, Windows and Linux which you can buy also directly or at a competitor such as GOG, if the developer chooses to.
To sell into the iOS ecosystem as a Dev you can only do it via Apple so far. Epic wants to sell their own iOS multiplayer games and in app lootboxes without forking 30% to Apple.
I do wonder: anybody knows if Amazon pays Apple for the things I buy through their iOS app?