Both Apple and Valve (Steam) allow similar arrangements, although not precisely the same. EA games on Steam, and Ubisoft games on Steam before they left Steam, require installing Origin and UPlay respectively. But don't you find it notable that Epic does not distribute Uplay on its own, unbundled from purchases?

On the Apple side, it's ok for applications to require the user make an account on services like Uplay (I believe several Ubisoft games require this; and Fortnite requires the Epic equivalent). You can then use that account to buy stuff on the stores. It's not an exact parallel because there are no (non-jailbreak) external app stores on the iPhone, but it's a close parallel.