It's not just bullshit—it's diabolical.
On your iOS device, you are logged in with one Apple ID at a time. This Apple identity is the one that makes In-App Purchases ("IAPs"). So if a user makes a purchase in your app signed in as "bob", and then logs out of your app and signs in as "alice", they still have that IAP. Therefore, the simplest, most correct way to have an app with IAPs and third party auth is to exclusively use Sign In With Apple, because Sign In With Apple is tied to your device's currently signed in Apple ID and all of its IAPs.
https://stackoverflow.com/questions/63272699/how-do-you-hand...
I considered removing my other third party auth methods but have kept them in to see how it goes... :|