You need to enable Bluetooth as a method of connection in the app settings (and can turn off wifi and data there).

The phones can be in airplane mode but with Bluetooth turned back on (as you would to use earbuds).

I don’t recall pairing the phones, but there is a “connect via Bluetooth” option on each chat that might be doing that automatically.

You link accounts to each other by scanning qr codes.

It does have a group chat but I haven’t used it, so I don’t know if that works with Bluetooth alone.

I just tried testing this with an old phone of mine, but can’t get it to work right now (maybe because it has Graphene os?), but I have actually used it on flights in the past.