Not everything. The transparency prompt would still appear, but Facebook wouldn't need to show users that Privacy Label in the App Store. They could also access restricted iOS Entitlements that the App Store would not permit. These entitlements can do things such as "read data from other apps" or "see what apps are installed in the system" or other functionality that the App Store doesn't allow under almost any circumstances.

If sideloading was enabled, Facebook could claim use of those entitlements. iOS could potentially show a prompt forcing users to OK their use, but Facebook would obviously hope most people just tap the button without thinking.

further toward the ideal consumer outcome, the apple app store would be compelled to compete with any other ios app store (e.g., cydia) but apple would be allowed to be as stringent about security and privacy of the device as they’d like (promoting competition in the mobile phone market, in addition to the apps/app store market). then, facebook couldn’t circumvent the protections that consumers want in their mobile devices by hopping to another app store.