They don't need to decrypt it. They control both sides, they can just track you using the app and get the information they need before encrypting. 

Similar to how WhatsApp/Telegram can get information from you even though they are "end to end" encrypted. They can just monitor what you type (which they do).

If the app is not opensource so that you can check it and if you can't change the server that is storing this info, they can track anything they want and you will never know.