Without that clause, your approach wouldn't work, because what incentive would Facebook even have to release it on the Apple App Store and follow Apple's tighter privacy/anti-tracking rules, when they can just release it on the Facebook App Store for iOS (or whatever else they decide to call it or, alternatively, another third-party app store)?

Sure, having third-party app stores helps smaller devs. But it also unchains all the anti-tracking and privacy shackles from the tech giants like FB who don't care which app store they are on (as long as they can set their own rules), because FB/Instagram/etc. users will follow to whatever app store their app is on (no matter how much or how little privacy protection that specific app store is willing to enforce on FB).