Fb Messenger working without cellular/wifi data, how?

I have iphone 7, it is up to date IOS wise.. Recently I turned my cellular data off for the FB messenger app in IOS settings. I also did not have access to wifi of any kind. However, yesterday I was messaging some friends back and forth on the app. I didnt dawn on me until tonight that it should have been physically impossible for me to do that. I checked and yes the cellular data usage is still off in IOS settings. Can someone help me explain this? I'm confused. I should note that I do have cellular data turned on for fb in the iOS settings but I'm not sure if that matters because the app is its own thing right? Any help figuring this out would be great. Thanks!