A more "air gap"-ish concept would have been a camera on the Firefly. Take a pic of a QR code on the phone, sign on the device, use the phone to take a pic of the output QR.
Possible problem: The attacker sends a transaction at the same time (or just before) a legit one is sent.
Not a big deal: The user is asked to send a specific quantity to a specific address on the screen. If somehow the user didn't check or the attacker fooled him with the same quantity etc, the picture still has to be taken and check it is the same transaction. Additionally, the wireless communication can have a second authentication factor.