https://pub.dev/documentation/firebase_core/latest/firebase_core/FirebaseOptions/androidClientId.html

The documentation states

The Android client ID from the Firebase Console, for example "12345.apps.googleusercontent.com."

This value is used by iOS only.

If this is correct and it is only for iOS some additional info of what it is for would be great, but I would guess it was just copied from https://pub.dev/documentation/firebase_core/latest/firebase_core/FirebaseOptions/iosClientId.html and the bottom line was missed.