I suspect it went something like this. "Please turn on the airplane mode now." Phone put into airplane mode. Phone disables BT. Watch loses connection. Because of questionable engineering choices, it automatically enters pairing mode and looks for another known device while also broadcasting its name. Most BT and WiFi devices in the area are turned off so the list of broadcasting devices is very short. The plane crew (I assume) manually check for strong 2.4G signals as part of take off procedures. "The Bomb" is sticking out like a sore thumb among the 5-10 other Fitbits and JBLs.

Importantly, the set of circumstances is so specific it couldn't have happened anywhere that is not an inside of an airplane about to take off - but also it would've happened inside every airplane about to take off.

There are way fewer Bluetooth signals active at once on an airplane then there are in an airport. There is also the entire duration of the flight for it to be discovered. Also take in to account that most people are not fidgeting with their device settings when they are walking around an airport, they are trying to get through security and get to their gate. Once you are on a plane that’s when you usually stop and start setting up your devices, such as connecting headphones.