Why would two heavily-shielded devices in physically independent systems at different long distances away from a puny transmitter react at all, let alone in the same way, to RF from that puny transmitter?
Cell phones are not puny transmitters and their signals are well strong enough inside an aircraft.
FADEC systems still have input from the cockpit because FADEC has top accommodate commands from the pilots.
The biggest idiots are those who don't dare to ask what if for fear of looking stupid.
That doesn't mean you can program your TV by using your vacuum cleaner.
Analog interference from a vacumn cleaner is not a danger to a digital system, but a string of digital signals inside a cabin from a digital cell phone to some component of the aircraft's digital network is an entirely different matter.