Did you consider that the app could simply be buggy? Or do some misread of data?
Perhaps it multiplied the actual amount used, so when two of your phones communicated to exchange content, it got it wrong.
I don't know how to phase it, lack of proper English vocabulary, so forgive me for that poor attempt.
Think of the phone as a Self Driving Car. For a car to turn on, engage gear and drive out your garage, would take way too many mechanics and "magic" to make it happen. Same apply with two devices communicating. This isn't a simple process, whether using multi channel binding or radio, such communication is fairly battery consuming, so the device has lots of software features to prevent it from running for too long, and especially on its own. It's just really hard for me to imagine, the phone doing that without asking permissions from you and the other device, and from software perspective, almost impossible as paring sessions are not Bluetooth like, require repair each time you want to send data across.
I seriously thing, the app is wrong and the phone never did any data traffic. You have no Server/ISP logs, Android itself has no memory of using it and overall no trace of any data being used, beside that one app.
I'm almost certain at this point, your app has a problem.