I don't know about the internals of Authy, but Bluetooth encrypts by default and so far it has not been shown to be terrible (if you use a sufficiently random key while pairing).

So even if Authy does nothing special to send the data encrypted, BT itself will ensure that it's safe. Minus, of course, some malware that's running and inspecting the application in-memory or just watching the clipboard, but no encryption on earth will help you there.