That's...not how APIs work.
They've removed APIs that Facebook was using (read: abusing) in PushKit in iOS 13. Their choices from there are to either continue linking against the iOS 12 SDK for as long as they're allowed to, which means they would never get to use the new features in iOS 13, or to use private API — if it exists and if they can discover it — to recreate the previous functionality, which would then require them to distribute outside the App Store. It's no longer provided by PushKit, so it's then up to Facebook to provide that functionality for themselves.
In other words, yes, this change would affect them even if they distributed outside the App Store.
For the poor souls who work with software
https://developer.apple.com/documentation/pushkit
(available since ios 8)
My guess is they cant use ios12 or older sdks but rather ios13 sdk which should have been backwards compatible BUT this is the update/heads up here - its NOT going to be
Wonder what is being tightened here from the API pov. https://developer.apple.com/documentation/pushkit/pkpushpayload seems to be the one that could be tightened unless the exploit is - apps can collect data when they receive a pushKit notification which otherwise wouldnt be possible if background refresh is disabled for them. That would be my guess.