If I delete all app data and reinstall the game, the old progress is immediately loaded on startup. So they must have some way to uniquely detect the phone.
There must be a unique App ID that is sent on the first launch. If there were a way to change that ID, the game would reset.
But that’s just a thought, I’m not a programmer...
@Edit:
Oh, wait. It can’t be a hardware-based ID.
When I use modded APKs from different websites, the game always starts from the beginning. So it must be a unique App ID.