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.