Just for a data point: I just tried with my iPhone SE, iOS 13.3: was connected to my wifi and a Bluetooth device. When I turned on flight mode (Aeroplane Mode in Applespeak), the wifi went off but the Bluetooth stayed connected. That surprised me, as I thought flight mode cut out all transmissions. If it is a change in behaviour, would explain why an airline would get fussy when there's a genuine possibility of interference.