Uh, it's not exclusively for Apple devices.
'Sign in with Apple' was designed to work on multiple platforms, specifically including Android.
https://firebase.google.com/docs/auth/android/apple
I'm literally holding a Xiaomi Mi Mix 2 right now, running Android 10, with an app open that says "Sign in with Apple" (Adobe Reader).
The Byte developers chose to not use it on Android, and then chose to not allow a user to link their sign-in methods (Google & Apple). Allowing a user to link their accounts would let them sign in with Google and access an account created with Apple.
This is 100% a choice by Byte, and I don't understand why.